75. Sort Colors

• Difficulty: Medium

• Topics: Array, Two Pointers, Sort

• Similar Questions:

  • Sort List

  • Wiggle Sort

  • Wiggle Sort II

Problem:

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?

Solutions:

class Solution {
public:
    void sortColors(vector<int>& nums) {
        int left = 0;
        int right = nums.size() - 1;

        for (int i = left; i <= right; ) { // not include ++i to continue swap if necessary; otherwise [1, 2, 0] fails; 
            if (nums[i] == 0 && i != left) { // remember the second check; otherwise [] would fail
                swap(nums[i], nums[left]);
                ++left;
            } else if (nums[i] == 2 && i != right) { // remember the second check; otherwise [] would fail
                swap(nums[i], nums[right]);
                --right;
            } else {
                ++i;
            }
        }
    }
};