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  • Leetcode-75.Sort Colors

    地址:https://leetcode.com/problems/sort-colors/

    描述:

    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.

    实例:

    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?

    方法1:

     // 时间复杂度O(n)
        // 空间复杂度 O(k)
        public void sortColors(int[] nums) {
            int[] count = new int[3];
    
            for (int i = 0; i < nums.length; i++) {
                if (nums[i] >= 0 && nums[i] <= 2) {
                    count[nums[i]]++;
                }
            }
    
            int index = 0;
            for (int i = 0; i < count[0]; i++) {
                nums[index++] = 0;
            }
    
            for (int i = 0; i < count[1]; i++) {
                nums[index++] = 1;
            }
    
            for (int i = 0; i < count[2]; i++) {
                nums[index++] = 2;
            }
        }

    方法2:

        // 时间复杂度O(n)
        // 空间复杂度 O(1)
        public void sortColors_2(int[] nums) {
            int zero = -1;  //nums[0,zero]是0
            int two = nums.length; // nums[two...n-1]是2
    
            for (int i = 0; i < two; i++) {
                if (nums[i] == 1) {
                    i++;
                } else if (nums[i] == 2) {
                    swap(nums, i, --two);
                } else {
                    if (nums[i] == 0) {
                        swap(nums, ++zero, i++);
                    }
                }
            }
        }
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  • 原文地址:https://www.cnblogs.com/zimengfang/p/10793446.html
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