solves bnary tree level order traversla II

Author: anishLearnsToCode

src/BinaryTreeLevelOrderTraversalII.java

@@ -0,0 +1,58 @@
+import java.util.ArrayList;
+import java.util.Collections;
+import java.util.LinkedList;
+import java.util.List;
+import java.util.Queue;
+
+public class BinaryTreeLevelOrderTraversalII {
+    private static class TreeNode {
+        int val;
+        TreeNode left;
+        TreeNode right;
+    }
+
+    private static class Pair {
+        TreeNode tree;
+        int level;
+
+        Pair(TreeNode tree, int level) {
+            this.tree = tree;
+            this.level = level;
+        }
+    }
+
+    public List<List<Integer>> levelOrderBottom(TreeNode root) {
+        List<List<Integer>> levels = new ArrayList<>();
+        if (root == null) {
+            return levels;
+        }
+
+        LinkedList<Pair> queue = new LinkedList<>();
+        queue.add(new Pair(root, 0));
+        List<Integer> level = new ArrayList<>();
+        int currentLevel = 0;
+
+        while (!queue.isEmpty()) {
+            Pair pair = queue.pop();
+            if (pair.level > currentLevel) {
+                levels.add(level);
+                level = new ArrayList<>();
+                currentLevel = pair.level;
+            }
+            level.add(pair.tree.val);
+            addChildrenToQueue(pair, queue);
+        }
+        levels.add(level);
+        Collections.reverse(levels);
+        return levels;
+    }
+
+    private static void addChildrenToQueue(Pair pair, Queue<Pair> queue) {
+        if (pair.tree.left != null) {
+            queue.add(new Pair(pair.tree.left, pair.level + 1));
+        }
+        if (pair.tree.right != null) {
+            queue.add(new Pair(pair.tree.right, pair.level + 1));
+        }
+    }
+}