Java O(n) solution with explanation

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    1. Find the index "sortIdx" after which the numbers are sorted in descending order.
    2. Find the index "swapIdx" which is the least number greater than nums[sortIdx-1] after sortIdx. (That is the last index greater than nums[sortIdx-1] since the subarray after sortIdx is sorted in descending order.)
    3. Swap sortIdx-1 and swapIdx-1.
    4. Reverse the subarray after sortIdx.
    public class Solution {
        public void nextPermutation(int[] nums) {
            if(nums==null || nums.length<=1) return;
            final int N = nums.length;
            int sortIdx = N-1; // numbers after sortIdx (include) is sorted in descending order 
            for(int i=N-1; i>=1 && nums[i]<=nums[i-1]; i--){
                int swapIdx = N;
                for(int i=N-1; i>=sortIdx && nums[i]<=nums[sortIdx-1]; i--){
                    swapIdx = i;
                swap(nums, sortIdx-1, swapIdx-1);
            reverse(nums, sortIdx, N-1);
        private void swap(int[] nums, int i, int j){
            int temp = nums[i];
            nums[i] = nums[j];
            nums[j] = temp;
        private void reverse(int[] nums, int i, int j){
                swap(nums, i, j);
                i++; j--;

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