75. Sort Colors
Description
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.
Intuition: The problem is known as Dutch National Flag Problem and first was proposed by Edsger W. Dijkstra. The idea is to attribute a color to each number and then to arrange them following the order of colors on the Dutch flag.
Note: You are not suppose to use the library's sort function for this problem.
Constraints
Approach
Links
GeeksforGeeks
ProgramCreek
Examples
Input: [2, 0, 2, 1, 1, 0]
Output: [0, 0, 1, 1, 2, 2]
Solutions
/**
* Time complexity : O(N) since it's one pass along the array of length N.
* Space complexity : O(1) since it's a constant space solution.
*/
class Solution {
public void sortColors(int[] nums) {
int left = 0, right = nums.length-1;
int curr = 0, tmp;
while(curr <= right) {
if(nums[curr] == 0) {
tmp = nums[left];
nums[left++] = nums[curr];
nums[curr++] = tmp;
} else if(nums[curr] == 2) {
tmp = nums[right];
nums[right--] = nums[curr];
nums[curr] = tmp;
} else {
curr++;
}
}
}
}
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?
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