Lintcode 148 Sort Colors (双指针好题)

148 · Sort Colors

Description
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.
You are not suppose to use the library’s sort function for this problem.
You should do it in-place (sort numbers in the original array).
You are not allowed to use counting sort to solve this problem.

Example 1

Input : [1, 0, 1, 2]
Output : [0, 1, 1, 2]
Explanation : sort it in-place
Challenge
Could you come up with an one-pass algorithm using only O(1) space?

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Related Knowledge
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解法1：双指针

class Solution {
public:
    /**
     * @param nums: A list of integer which is 0, 1 or 2
     * @return: nothing
     */
    void sortColors(vector<int> &nums) {
        int n = nums.size();
        //RED 0, WHITE 1, BLUE 2
        //Before each iteration in white loop:
        //  [0..red) are red 0
        //  [red..index) are white 1
        //  [index..blue] are unknown
        //  (blue..n-1] are blue 2
        int red = 0, index = 0, blue = n - 1;
        while (index <= blue) {  //注意：这里不是index
            if (nums[index] == 0) {
                swap(nums[red], nums[index]); //nums[red]已经扫描过，一定是1，所以可以动index
                red++; index++;
            } else if (nums[index] == 1) {
                index++;
            } else { //nums[index] == 2
                swap(nums[blue], nums[index]); //nums[blue]还没有扫描过，可能是0,1,2，所以不能动index
                blue--;
            }
        }
        return;
    }
};