[LeetCode] Sort Colors

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.

解题思路：
1. 最开始做的是统计0,1,2的数目。然后要扫2遍
2. 这题只有3个数，想到可以用3个指针分别指向一个数字，2边的指针s, e分别指向0，和2。中间的指针 i扫描 s-e 数。碰到0，则swap( i, s)，移动s . 碰到2, swap(i, e), 移动e.

第二个方法，注意移动i, s, e的移动。 由于等于2的时候， i 会和e 交换，也就是当i扫描到0的时候，i 交换的数一定为1，所以交换后，i也要移动。

Java Code:

 public void sortColors(int[] A) {
        int[] count = new int[3];

        for (int i = 0; i < A.length; i++) {
            switch (A[i]) {
                case 0:
                    count[0]++;
                    continue;
                case 1:
                    count[1]++;
                    continue;
                case 2:
                    count[2]++;
                    continue;
            }
        }

        for (int i = 0; i < A.length; i++) {
            if (i < count[0]) {
                A[i] = 0;
            } else if (i < count[0] + count[1]) {
                A[i] = 1;
            } else {
                A[i] = 2;
            }
        }

  public void sortColors(int[] A) {
        int s = 0;
        int e = A.length - 1;

        int i = s;

        while (i <= e) {
            if (A[i] == 0) {
                A[i] = A[s];
                A[s] = 0;
                s++;
                i++;
                continue;
            }
            
            if (A[i] == 2) {
                A[i] = A[e];
                A[e] = 2;
                e--;
                continue;
            }
                i++;
        }
    }