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.

Here, we will use the integers 0, 1, and 2 to represent the color red, white, and blue respectively.

Notice
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).

Example
Given [1, 0, 1, 2], sort it in-place to [0, 1, 1, 2].

Challenge:
(avoiding statistics sort)
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 an one-pass algorithm using only constant space?

Link

Lintcode_ladder

Method

1. from start to end find 0, put it to the start(swap),
   iterator + 1, start + 1
   find 2, put it to the end(swap),
   end - 1
   find 1, do nothing, iterator + 1
2. x

Example

1. 1

class Solution{
public:
    /**
     * @param nums: A list of integer which is 0, 1 or 2 
     * @return: nothing
     */    
    void sortColors(vector<int> &nums) {
        // write your code here
        if (nums.empty()) {
            return;
        }
        // ------------------------------------------------
        // Naive version
        int n = nums.size();
        int start = 0;
        int end = n-1;
        int i = 1;
        while (start < i && i < n) {
            // for red to start
            if (nums[i] == 0) {
                swap(nums[i], nums[start]);
                start++;
                while (nums[i] == 0 && start < i) {
                    swap(nums[i], nums[start]);
                    start++;
                }
            }
            ++i;
        }
        i = start;
        //cout << start << "---\n";
        while (i < end) {
            if (nums[i] == 2) {
                swap(nums[i], nums[end]);
                end--;
                while (nums[i] == 2 && i < end) {
                    swap(nums[i], nums[end]);
                    end--;
                }
            } 
            ++i;
        }
        // ----------------------------------------------
        // opted version
        int n = nums.size();
        int start = 0;
        int end = n-1;
        int i = 0;
        while (i <= end) {
            if (nums[i] == 0) {
                swap(nums[i], nums[start]);
                start++;
                ++i;
            } else if (nums[i] == 2) {
                swap(nums[i], nums[end]);
                --end;
            } else {
                ++i;
            }
        }
        return;
    }
};

Similar problems

x

Tags

x