949-largest-time-for-given-digits

class Solution {
    private int max = -1;
    public String largestTimeFromDigits(int[] A) {
        
        dfs(A,0);
        if(max ==-1){
            return "";
            
        }else{
            return String.format("%02d:%02d",max/100,max%100);
        }
        
    }
    
   private void dfs(int[]A, int start){
        
        if(start==A.length){
            this.getMaxTime(A);
            return;
        }
       //recurse
       for(int i=start; i<A.length; i++){
           
           this.swap(A,i,start);
           this.dfs(A,start+1);
           this.swap(A,i,start);
       }
       
        
    }
    private void getMaxTime(int[] A){
        int hour = A[0]*10 + A[1];
        int mins = A[2]*10 + A[3];
        
        if(hour<24 && mins <60) {
            //time is valid. check if current time is higher than prev highest
            
            max = Math.max(max, hour*100+mins);
        }
    }

    private void swap(int[] A, int x, int y){
        if(x!=y){
        int temp = A[x];
        A[x] = A[y];
        A[y] = temp;
        }

    }
    
    
}