LeetCode 31. Next Permutation

Posted 2020-07-13

Problem:

https://leetcode.com/problems/next-permutation/

Implement next permutation, which rearranges numbers into the lexicographically next greater permutation of numbers.

If such arrangement is not possible, it must rearrange it as the lowest possible order (ie, sorted in ascending order).

The replacement must be in-place, do not allocate extra memory.

Here are some examples. Inputs are in the left-hand column and its corresponding outputs are in the right-hand column.
1,2,3 → 1,3,2
3,2,1 → 1,2,3
1,1,5 → 1,5,1

Thought:

　　from end to begin, find the first number that have number greater than it after, then swap the number with the minimum greater number, the sort the array after the number.

　　e.g    2 6 3 4 3 1     find  arr[2] = 3, the swap it with arr[3] = 4, sort arr[3] to arr[5]

 

Code  C++:

class Solution {
public:
    void nextPermutation(vector<int>& nums) {
        if (nums.size() <= 1)
            return;
        
        for (int i = nums.size() - 2; i >= 0; i--) {
            int greater_min = i;//greater_min point to the minimum number greater than nums[i]
            
            for (int j = nums.size() - 1; j > i; j--) {//get greater_min
                if (nums[j] > nums[i]) {
                    if (greater_min == i) {
                        greater_min = j;
                        continue;
                    }
                    greater_min  = nums[j] < nums[greater_min] ? j : greater_min;
                }
            }
            
            if (greater_min != i) {
                swap(nums[i], nums[greater_min]);
                sort(nums.begin() + i + 1, nums.end());
                return;
            }
        }
        sort(nums.begin(), nums.end());
        return;
    }
};