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anishLearnsToCodeanishLearnsToCode
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solves #51: N-Queens in java
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‎README.md‎

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| 48 | [Rotate Image](https://leetcode.com/problems/rotate-image) | [![Java](assets/java.png)](src/RotateImage.java) | |
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| 49 | [Group Anagrams](https://leetcode.com/problems/group-anagrams) | [![Java](assets/java.png)](src/GroupAnagrams.java) | |
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| 50 | [Pow(x,n)](https://leetcode.com/problems/powx-n) | [![Java](assets/java.png)](src/Powxn.java) | |
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| 51 | [N-Queens](https://leetcode.com/problems/n-queens) | [![Java](assets/java.png)](src/NQueens.java) | |
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| 53 | [Maximum SubArray](https://leetcode.com/problems/maximum-subarray) | [![Java](assets/java.png)](src/MaximumSubArray.java) [![Python](assets/python.png)](python/maximum_sum_subarray.py) | |
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| 54 | [Spiral Matrix](https://leetcode.com/problems/spiral-matrix) | [![Java](assets/java.png)](src/SpiralMatrix.java) | |
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| 55 | [Jump Game](https://leetcode.com/problems/jump-game) | [![Java](assets/java.png)](src/JumpGame.java) | |

‎src/CombinationSum.java‎

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// https://leetcode.com/problems/combination-sum
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// T: O(|candidates| ^ target)
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// S: O(target + candidate ^ 2) // not sure tho --> if anyone has any idea please feel free to contact me
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// M = smallest candidate
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// T: O(|candidates| ^ (target / M))
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// S: O(target / M) // not sure tho --> if anyone has any idea please feel free to contact me
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import java.util.ArrayList;
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import java.util.List;

‎src/HelloWorld.java‎

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import java.util.Set;
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public class HelloWorld {
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// T: O(V + E) S: O(V + E)
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public int[] findOrder(int numCourses, int[][] prerequisites) {
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final Map<Integer, Set<Integer>> graph = createGraph(prerequisites);
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final int[] inDegree = computeInDegree(numCourses, prerequisites);
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return orderOfCourses(graph, inDegree);
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public List<List<Integer>> combinationSum(int[] candidates, int target) {
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final Set<List<Integer>> result = new HashSet<>();
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computeSums(candidates, target, result, 0, new ArrayList<>(), 0);
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return new ArrayList<>(result);
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}
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// T: O(V + E) S: O(V)
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private static int[] orderOfCourses(Map<Integer, Set<Integer>> graph, int[] inDegree) {
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final Queue<Integer> queue = new PriorityQueue<>();
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final List<Integer> order = new ArrayList<>();
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addAll0InDegreeToQueue(queue, inDegree);
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while (!queue.isEmpty()) {
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final int vertex = queue.poll();
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order.add(vertex);
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for (int neighbour : graph.getOrDefault(vertex, new HashSet<>())) {
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inDegree[neighbour]--;
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if (inDegree[neighbour] == 0) {
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queue.add(neighbour);
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}
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}
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graph.remove(vertex);
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private static void computeSums(int[] candidates, int target, Set<List<Integer>> result, int currentSum, List<Integer> current, int i) {
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if (currentSum > target || i >= candidates.length) {
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return;
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}
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if (order.size() != inDegree.length) {
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return new int[] {};
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if (currentSum == target) {
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result.add(new ArrayList<>(current));
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return;
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}
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return toArray(order);
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}
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private static int[] toArray(List<Integer> list) {
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final int[] array = new int[list.size()];
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for (int i = 0 ; i < array.length ; i++) {
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array[i] = list.get(i);
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}
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return array;
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}
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// T: O(E) S: O(1)
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private static void addAll0InDegreeToQueue(Queue<Integer> queue, int[] inDegree) {
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for (int i = 0 ; i < inDegree.length ; i++) {
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if (inDegree[i] == 0) {
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queue.add(i);
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}
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}
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}
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// T: O(E) S: O(E)
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private static int[] computeInDegree(int n, int[][] edges) {
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final int[] inDegree = new int[n];
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for (int[] edge : edges) {
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final int to = edge[0];
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inDegree[to]++;
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}
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return inDegree;
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}
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// T: O(|E|), S: O(E)
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private static Map<Integer, Set<Integer>> createGraph(int[][] edges) {
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final Map<Integer, Set<Integer>> graph = new HashMap<>();
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for (int[] edge : edges) {
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final int to = edge[0], from = edge[1];
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final Set<Integer> set = graph.getOrDefault(from, new HashSet<>());
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set.add(to);
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graph.putIfAbsent(from, set);
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}
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return graph;
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computeSums(candidates, target, result, currentSum, current, i + 1);
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current.add(candidates[i]);
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computeSums(candidates, target, result, currentSum + candidates[i], current, i);
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computeSums(candidates, target, result, currentSum + candidates[i], current, i + 1);
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current.removeLast();
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}
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}

‎src/NQueens.java‎

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// https://leetcode.com/problems/n-queens
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// T: O(N!)
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// S: O(N^2)
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import java.util.ArrayList;
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import java.util.List;
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public class NQueens {
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private static boolean[] rows, columns;
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11+
public static List<List<String>> solveNQueens(int n) {
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final List<List<String>> result = new ArrayList<>();
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final List<String> board = getEmptyBoard(n);
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rows = new boolean[n];
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columns = new boolean[n];
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nQueens(0, n, result, board, 0);
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return result;
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}
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private static void nQueens(int row, int n, List<List<String>> result, List<String> board, int queens) {
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if (row == n) {
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if (queens == n) {
23+
result.add(new ArrayList<>(board));
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}
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return;
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}
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for (int column = 0 ; column < n ; column++) {
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if (canPlace(board, row, column)) {
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placeQueen(board, row, column);
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nQueens(row + 1, n, result, board, queens + 1);
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removeQueen(board, row, column);
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}
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}
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}
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private static void placeQueen(List<String> board, int row, int column) {
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board.set(
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row,
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board.get(row).substring(0, column) + 'Q' + board.get(row).substring(column + 1)
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);
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rows[row] = true;
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columns[column] = true;
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}
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46+
private static void removeQueen(List<String> board, int row, int column) {
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board.set(
48+
row,
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board.get(row).substring(0, column) + '.' + board.get(row).substring(column + 1)
50+
);
51+
rows[row] = false;
52+
columns[column] = false;
53+
}
54+
55+
private static boolean canPlace(List<String> board, int row, int column) {
56+
return !rows[row] && !columns[column] && !queenInLeftDiagonal(board, row, column)
57+
&& !queenInRightDiagonal(board, row, column);
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}
59+
60+
private static boolean queenInLeftDiagonal(List<String> board, int row, int column) {
61+
for (int i = row - 1, j = column - 1 ; i >= 0 && j >= 0 ; i--, j--) {
62+
if (board.get(i).charAt(j) == 'Q') {
63+
return true;
64+
}
65+
}
66+
for (int i = row + 1, j = column + 1 ; i < board.size() && j < board.size() ; i++, j++) {
67+
if (board.get(i).charAt(j) == 'Q') {
68+
return true;
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}
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}
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return false;
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}
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private static boolean queenInRightDiagonal(List<String> board, int row, int column) {
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for (int i = row - 1, j = column + 1 ; i >= 0 && j < board.size() ; i--, j++) {
76+
if (board.get(i).charAt(j) == 'Q') {
77+
return true;
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}
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}
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for (int i = row + 1, j = column - 1 ; i < board.size() && j >= 0 ; i++, j--) {
81+
if (board.get(i).charAt(j) == 'Q') {
82+
return true;
83+
}
84+
}
85+
return false;
86+
}
87+
88+
private static List<String> getEmptyBoard(int n) {
89+
final List<String> board = new ArrayList<>();
90+
final String line = ".".repeat(n);
91+
for (int i = 0 ; i < n ; i++) {
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board.add(line);
93+
}
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return board;
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}
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}

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