# 75. Sort Colors (Medium)

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

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]```

• 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?

## Solutions

``````class Solution {
public void sortColors(int[] nums) {
// slot for 0 starts from 0 and moves forward
int slotFor0 = 0;

// slot for 2 starts from len-1 and moves reversely
int sloptFor2 = nums.length - 1;

int ptr = 0;

while (ptr <= sloptFor2) {
if (nums[ptr] == 0) {
swap(nums, slotFor0, ptr);

// Here, the reason why ptr does not step back to check element
// from slotFor0 is that it already checked since ptr always moves
// faster than slotFor0

slotFor0++;

} else if (nums[ptr] == 2) {
swap(nums, ptr, sloptFor2);

// Step back to the newly swapped element from slotFor2 because it has not
// been checked yet.
ptr--;

sloptFor2--;
}

ptr++;
}
}

private void swap(int[] nums, int i, int j) {
int temp = nums[i];
nums[i] = nums[j];
nums[j] = temp;
}
}
``````