add solution of problem 75: sort colors

Author: ppulse

SortColors75/README.md

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+Given an array with *n* objects colored red, white or blue, sort them so that objects of the same color are adjacent, with the colors in the order red, white and blue.
+
+Here, we will use the integers 0, 1, and 2 to represent the color red, white, and blue respectively.
+
+Note:
+You are not suppose to use the library's sort function for this problem.

SortColors75/Solution.java

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+public class Solution {
+    public void sortColors(int[] nums) {
+        if (nums == null)
+            return;
+        
+        int left = 0;
+        int right = nums.length - 1;
+        
+        //step one
+        int idxFirstOne = sort(nums, left, right, 0);
+        
+        //step two
+        right = nums.length - 1;
+        sort(nums, idxFirstOne, right, 1);
+    }
+    
+    private final int sort(int nums[], int left, int right, int pivot) {
+        while (left < right) {
+            while (left < right && nums[left] == pivot) {
+                left++;
+            }
+            
+            while (left < right && nums[right] != pivot) {
+                right--;
+            }
+            
+            if (left < right) {
+                int tmp = nums[left];
+                nums[left] = nums[right];
+                nums[right] = tmp;
+            }
+        }
+        
+        return left;
+    }
+}