leetcode75. Sort Colors

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本文介绍了一种将红、白、蓝三色对象按顺序排列的算法。通过使用整数0、1和2分别代表红、白、蓝三种颜色，实现了一次遍历并仅使用常量空间的解决方案。

75. 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.

click to show follow up.

Follow up:
A rather straight forward solution is a two-pass algorithm using counting sort.
First, iterate the array counting number of 0’s, 1’s, and 2’s, then overwrite array with total number of 0’s, then 1’s and followed by 2’s.

Could you come up with an one-pass algorithm using only constant space?

解法

0放到最左边，left记录最左边的位置，存放一个0之后++,cur记录当前的位置，存放一个0之后加1，当前值为1时加1，right记录最右边的位置，存放一个2之后–。直到cur<=right.

public class Solution {
    public void sortColors(int[] nums) {
        if (nums == null || nums.length == 0) {
            return;
        }
        int left = 0, curr = 0, right = nums.length - 1;
        while (curr <= right) {
            if (nums[curr] == 0) {
                swap(nums, left++, curr++);
            } else if (nums[curr] == 2) {
                swap(nums, curr, right--);
            } else {
                curr++;
            }
        }
    }
    private void swap(int[] nums, int left, int right) {
        int temp = nums[left];
        nums[left] = nums[right];
        nums[right] = temp;
    }
}