【LeetCode】75. Sort Colors

Difficulty: Medium

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.

由于0,1,2非常紧凑，首先想到计数排序，但需要扫描两遍，不符合题目要求。

由于自由三种颜色，可以设置两个index，一个是red的index，一个是blue的index，两边往中间走。

时间复杂度O（n）

class Solution {
public:
void sortColors(vector<int>& A) {
int counts[3] = { 0 };
for (int i = 0; i < A.size(); i++)
counts[A[i]]++;
for (int i = 0, index = 0; i < 3; i++)
for (int j = 0; j < counts[i]; j++)
A[index++] = i;
}
};