75

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.

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

• O(1) space , one pass
  • first assign all element to 2, =>
  • then we check if the current element is smaller than 2, if it is, it could be 1 or 0, then assign the element to 1, =>
  • then we check if the current element is 0, if it is, then assign the element to 0;

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void (vector<int>& nums) {
    for(int i = 0, j = 0, k = 0; k < nums.size(); k++){
        int temp = nums[k];
        nums[k] = 2;
        if(temp < 2){
            nums[j] = 1;
            j++;
        }
        if(temp < 1){
            nums[i]  = 0;
            i++;
        }
    }
}