[leetcode 75] Sort Colors

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本文介绍了一种不使用库函数的排序算法，用于将包含三种颜色的数组按照颜色顺序排列，具体实现通过双指针技巧完成。

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.

class Solution {
public:
    void sortColors(int A[], int n) {
        if (n == 1) {
            return ;
        }
        int first = 0;
        int last = n-1;
        while (first <= last) {
            while (first < last && A[first] == 0) {
                first++;
            }
            while (last > first && A[last] != 0) {
                last--;
            }
            swap(A[first], A[last]);
            first++;
            last--;
        }
        first = 0;
        last = n - 1;
        while (first <= last) {
            while (first < last && A[first] < 2) {
                first++;
            }
            while (last > first && A[last] == 2) {
                last--;
            }
            swap(A[first], A[last]);
            first++;
            last--;
        }
    }
};

思路2：用两个index分别记录红色和蓝色的位置

class Solution {
public:
    void sortColors(int A[], int n) {
        if (n == 1) {
            return ;
        }
        int red = 0;
        int blue = n - 1;
        for (int i = 0; i < blue + 1;) {
            if (A[i] == 0) {
                swap(A[i++], A[red++]);
            } else if (A[i] == 2) {
                swap(A[i], A[blue--]);
            } else {
                i++;
            }
        }
    }
};