add 481

Author: fishercoder1534

README.md

@@ -31,6 +31,7 @@ Your ideas/fixes/algorithms are more than welcome!
 |592|[Fraction Addition and Subtraction](https://leetcode.com/problems/fraction-addition-and-subtraction/)|[Solution](../master/src/main/java/com/fishercoder/solutions/_592.java) | O(nlogx) |O(n) | Medium | Math
 |591|[Tag Validator](https://leetcode.com/problems/tag-validator/)|[Solution](../master/src/main/java/com/fishercoder/solutions/_591.java) | O(n) |O(n) | Hard | Stack, String
 |588|[Design In-Memory File System](https://leetcode.com/problems/design-in-memory-file-system/)|[Solution](../master/src/main/java/com/fishercoder/solutions/_588.java) | O(n) |O(h) | Hard | Trie, Design
+|587|[Erect the Fence](https://leetcode.com/problems/erect-the-fence/)|[Solution](../master/src/main/java/com/fishercoder/solutions/_587.java) | O(?) |O(?) | Hard | Geometry
 |583|[Delete Operation for Two Strings](https://leetcode.com/problems/delete-operation-for-two-strings/)|[Solution](../master/src/main/java/com/fishercoder/solutions/_583.java) | O(m*n) |O(m*n) could be optimized to O(n) | Medium | DP
 |582|[Kill Process](https://leetcode.com/problems/kill-process/)|[Solution](../master/src/main/java/com/fishercoder/solutions/_582.java) | O(n) |O(h) | Medium | Stack
 |581|[Shortest Unsorted Continuous Subarray](https://leetcode.com/problems/shortest-unsorted-continuous-subarray/)|[Solution](../master/src/main/java/com/fishercoder/solutions/_581.java) | O(n) |O(1) | Easy | Array, Sort
@@ -103,6 +104,7 @@ Your ideas/fixes/algorithms are more than welcome!
 |485|[Max Consecutive Ones](https://leetcode.com/problems/max-consecutive-ones/)|[Solution](../master/src/main/java/com/fishercoder/solutions/MaxConsecutiveOnes.java) | O(n) |O(1) | Easy| Array
 |484|[Find Permutation](https://leetcode.com/problems/find-permutation/)|[Solution](../master/src/main/java/com/fishercoder/solutions/_484.java) | O(n) |O(1) | Medium | Array, String, Greedy
 |482|[License Key Formatting](https://leetcode.com/problems/license-key-formatting/)|[Solution](../master/src/main/java/com/fishercoder/solutions/LicenseKeyFormatting.java) | O(n) |O(n) | Medium| 
+|481|[Magical String](https://leetcode.com/problems/magical-string/)|[Solution](../master/src/main/java/com/fishercoder/solutions/_481.java) | O(?) |O(?) | Medium| 
 |480|[Sliding Window Median](https://leetcode.com/problems/sliding-window-median/)|[Solution](../master/src/main/java/com/fishercoder/solutions/_480.java) | O(nlogk) |O(k) | Hard| Heap
 |477|[Total Hamming Distance](https://leetcode.com/problems/total-hamming-distance/)|[Solution](../master/src/main/java/com/fishercoder/solutions/TotalHammingDistance.java) | O(n) |O(1) | Medium| Bit Manipulation
 |476|[Number Complement](https://leetcode.com/problems/number-complement/)|[Solution](../master/src/main/java/com/fishercoder/solutions/NumberComplement.java) | O(n) |O(1) | Easy| Bit Manipulation

src/main/java/com/fishercoder/solutions/_481.java

@@ -0,0 +1,68 @@
+package com.fishercoder.solutions;
+
+/**
+ * 481. Magical String
+ *
+ * A magical string S consists of only '1' and '2' and obeys the following rules:
+
+ The string S is magical because concatenating the number of contiguous occurrences of characters '1' and '2' generates the string S itself.
+
+ The first few elements of string S is the following: S = "1221121221221121122……"
+
+ If we group the consecutive '1's and '2's in S, it will be:
+
+ 1 22 11 2 1 22 1 22 11 2 11 22 ......
+
+ and the occurrences of '1's or '2's in each group are:
+
+ 1 2	2 1 1 2 1 2 2 1 2 2 ......
+
+ You can see that the occurrence sequence above is the S itself.
+
+ Given an integer N as input, return the number of '1's in the first N number in the magical string S.
+
+ Note: N will not exceed 100,000.
+
+ Example 1:
+ Input: 6
+ Output: 3
+ Explanation: The first 6 elements of magical string S is "12211" and it contains three 1's, so return 3.
+ */
+public class _481 {
+
+    /**credit: https://discuss.leetcode.com/topic/74917/simple-java-solution-using-one-array-and-two-pointers
+     * Algorithm:
+
+     1. Create an int array a and initialize the first 3 elements with 1, 2, 2.
+     2. Create two pointers head and tail. head points to the number which will be used to generate new numbers.
+     tail points to the next empty position to put the new number. Then keep generating new numbers until tail >= n.
+     3. Need to create the array 1 element more than n to avoid overflow because the last round head might points to a number 2.
+     4. A trick to flip number back and forth between 1 and 2: num = num ^ 3*/
+    public int magicalString(int n) {
+        if (n <= 0) return 0;
+        if (n <= 3) return 1;
+
+        int[] a = new int[n + 1];
+        a[0] = 1;
+        a[1] = 2;
+        a[2] = 2;
+
+        int head = 2;
+        int tail = 3;
+        int num = 1;
+        int result = 1;
+
+        while (tail < n) {
+            for (int i = 0; i < a[head]; i++) {
+                a[tail] = num;
+                if (num == 1 && tail < n) result++;
+                tail++;
+            }
+            num = num ^ 3;
+            head++;
+        }
+
+        return result;
+    }
+
+}