Next Permutation

public class Solution {
    public void nextPermutation(int[] num) {
        if (num == null || num.length <= 1) {
            return;
        }        
        
        int partitionIndex = -1;
        // from right to left, find partitionNum
        for (int i = num.length - 2; i >= 0; i--) {
            if (num[i] < num[i + 1]) {
                partitionIndex = i;
            }
        }
        
        if (partitionIndex != -1) {
            int changeIndex = -1;
            for (int i = num.length - 1; i > partitionIndex; i--) {
                if (num[i] > num[partitionIndex]) {
                    // swap
                    int tmp = num[i];
                    num[i] = num[partitionIndex];
                    num[partitionIndex] = tmp;
                    
                    // keep the fowllowing increasing order
                    Arrays.sort(num, partitionIndex + 1, num.length);
                }
            }
        } else {
            Arrays.sort(num);
        }
        
    }
}