75. Sort Colors (1)

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Given an array 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.

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.

Example:

Input: [2,0,2,1,1,0]
Output: [0,0,1,1,2,2]

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 a one-pass algorithm using only constant space?

Solution

這題思路是設計兩個pointer分別指向0的下個位置(indexOf0)跟2的上個位置(indexOf2)。然後iteration每個element如果是0 就swap到indexOf0，如果是2就就swap到indexOf2.

值得注意的是i++何時要觸發，若是indexOf0==i ，則i++，因為經過這輪i已經處理過了，不用再來一次。

class Solution {
    public void sortColors(int[] nums) {
        int len = nums.length;
        int indexOf2 = len - 1;
        int indexOf0 = 0;
        int i = 0;
        while(i <= indexOf2){
            if(nums[i] == 2){
                nums[i] = nums[indexOf2];
                nums[indexOf2] = 2;
                indexOf2--;
            }
            else if(nums[i] == 0){
                nums[i] = nums[indexOf0];
                nums[indexOf0] = 0;
                if(i == indexOf0) i++;
                indexOf0++;
            }else{
                i++;
            }
            
        }
        
    }
}