# 75. Sort Colors

Given an array `nums` with `n` objects colored red, white, or blue, sort them in-place so that objects of the same color are adjacent, with the colors in the order red, white, and blue.

We will use the integers `0`, `1`, and `2` to represent the color red, white, and blue, respectively.

You must solve this problem without using the library’s sort function.

Example 1:

``````Input: nums = [2,0,2,1,1,0]
Output: [0,0,1,1,2,2]
``````

Example 2:

``````Input: nums = [2,0,1]
Output: [0,1,2]
``````

Constraints:

• `n == nums.length`
• `1 <= n <= 300`
• `nums[i]` is either `0`, `1`, or `2`.

Follow up: Could you come up with a one-pass algorithm using only constant extra space?

- 首先遍历一遍原数组，分别记录 0，1，2 的个数。
- 然后更新原数组，按个数分别赋上 0，1，2。

``````class Solution {
public:
void sortColors(vector<int>& nums) {
vector<int> colors(3);
for (int num : nums) ++colors[num];
for (int i = 0, cur = 0; i < 3; ++i) {
for (int j = 0; j < colors[i]; ++j) {
nums[cur++] = i;
}
}
}
};
``````

- 定义 red 指针指向开头位置，blue 指针指向末尾位置。

- 从头开始遍历原数组，如果遇到0，则交换该值和 red 指针指向的值，并将 red 指针后移一位。若遇到2，则交换该值和 blue 指针指向的值，并将 blue 指针前移一位。若遇到1，则继续遍历。

``````class Solution {
public:
void sortColors(vector<int>& nums) {
int red = 0, blue = (int)nums.size() - 1;
for (int i = 0; i <= blue; ++i) {
if (nums[i] == 0) {
swap(nums[i], nums[red++]);
} else if (nums[i] == 2) {
swap(nums[i--], nums[blue--]);
}
}
}
};
``````

``````class Solution {
public:
void sortColors(vector<int>& nums) {
int left = 0, right = (int)nums.size() - 1, cur = 0;
while (cur <= right) {
if (nums[cur] == 0) {
swap(nums[cur++], nums[left++]);
} else if (nums[cur] == 2) {
swap(nums[cur], nums[right--]);
} else {
++cur;
}
}
}
};
``````

Github 同步地址：

https://github.com/grandyang/leetcode/issues/75

Sort List

Wiggle Sort II

Wiggle Sort

https://leetcode.com/problems/sort-colors/

https://leetcode.com/problems/sort-colors/discuss/26500/Four-different-solutions

https://leetcode.com/problems/sort-colors/discuss/26472/Share-my-at-most-two-pass-constant-space-10-line-solution

https://leetcode.com/problems/sort-colors/discuss/26760/C%2B%2B-solution-in-8-lines%3A-an-instance-of-the-Dutch-national-flag-problem-by-Edsger-Dijkstra

LeetCode All in One 题目讲解汇总(持续更新中…)

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