128_leetcode_Sort Colors

1:

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.

1:注意特殊情況；2:遍歷數組一次；3:設置兩個指針，分別表示0和2的邊界，當從頭到後遍歷數組的時候，如果是1，直接遍歷下一個；如果爲0，與前面的數組進行交換；如果是2， 與後面的數字進行交換，注意循環結束的情況

    void sortColors(int A[], int n)
    {
        if(A == NULL || n <= 1)
        {
            return;
        }
        
        int leftIndex = -1;
        int rightIndex = n;
        int i = 0;
        
        while(i < rightIndex)
        {
            if(A[i] == 1 || i == leftIndex)
            {
                i++;
            }
            else if(A[i] == 0)
            {
                int tmp = A[i];
                A[i] = A[++leftIndex];
                A[leftIndex] = tmp;
            }
            else if(A[i] == 2)
            {
                int tmp = A[i];
                A[i] = A[--rightIndex];
                A[rightIndex] = tmp;
            }
        }
        return;
    }