75.sort-colors.java

/*
 * @lc app=leetcode id=75 lang=java
 *
 * [75] Sort Colors
 *
 * https://leetcode.com/problems/sort-colors/description/
 *
 * algorithms
 * Medium (42.30%)
 * Total Accepted:    330.5K
 * Total Submissions: 776.7K
 * 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) {
         if (nums == null || nums.length == 0) {
             return;
         }

         int redIndex = 0;
         int blueIndex = nums.length - 1;
         int cur = 0;

         while (cur <= blueIndex) {
             if (nums[cur] == 0) {
                 swap(nums, cur++, redIndex++);
             } else if (nums[cur] == 2) {
                 swap(nums, cur, blueIndex--);
             } else {
                 cur++;
             }
         }
     }

     private void swap(int[] nums, int i, int j) {
         int tmp = nums[i];
         nums[i] = nums[j];
         nums[j] = tmp;
     }
 }