75.sort-colors.java

/*
 * @lc app=leetcode id=75 lang=java
 *
 * [75] Sort Colors
 *
 * https://leetcode.com/problems/sort-colors/description/
 *
 * algorithms
 * Medium (43.01%)
 * Likes:    1899
 * Dislikes: 169
 * Total Accepted:    349.2K
 * Total Submissions: 810.9K
 * Testcase Example:  '[2,0,2,1,1,0]'
 *
 * 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?
 *
 *
 */
class Solution {
  public void sortColors(int[] nums) {
    int zero = 0;
    int second = nums.length - 1;

    for (int i = zero; i <= second; i++) {
      while (nums[i] == 2 && i < second) {
        swap(nums, i, second--);
      }
      while (nums[i] == 0 && i > zero) {
        swap(nums, i, zero++);
      }
    }
  }

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