Next Permutation

public class Solution {
    public void nextPermutation(int[] num) {
        // Note: The Solution object is instantiated only once and is reused by each test case.
        int length = num.length;
        int index = - 1;
        for(int i = length - 1; i >= 1; i--){
            if(num[i] > num[i - 1]){
                index = i;
                break;
            }
        }
        
        if(index == -1){
            reverseArray(num, 0, length - 1);
        } else {
            int biggerIndex = findBig(num[index - 1], index, num);
            swap(num, biggerIndex, index - 1);
            reverseArray(num, index, length - 1);
        }
    }
    
    public int findBig(int sentinal, int index, int[] num){
        int bigIndex = index;
        int bigValue = num[index];
        for(int i = index + 1; i < num.length; i++){
            if(num[i] > num[index - 1] && num[i] <= bigValue){
                bigValue = num[i];
                bigIndex = i;
            }
        }
        return bigIndex;
    }
    
    public void reverseArray(int[] num, int start, int end){
        while(start < end){
            swap(num, start, end);
            start ++;
            end --;
        }
    }
    
    public void swap(int[] num, int start, int end){
        int tmp = num[start];
        num[start] = num[end];
        num[end] = tmp;
    }
}