Backtracking.java

public class Backtracking {
    public static void changeArr(int arr[],int i,int val){
        // base case
        if(i == arr.length){
            pritnArr(arr);
            return;
        }
        //recursion
        arr[i] = val;
        changeArr(arr, i+1, val+1);  // function call step. After this func is completed in stack the last step of backtracking happens.
        arr[i] = arr[i] -2 ;    // backtracking step.
    }
    public static void pritnArr(int arr[]){
        for(int i=0;i<arr.length;i++){
            System.out.print(arr[i]+" ");
        }
        System.out.println();
    }

    public static void findSubsets(String str,String ans , int i){
        // base case
        if(i == str.length()){
            if(ans.length()==0){
                System.out.println("null");
            }
           else{
            System.out.println(ans);
           }
           return;
        }
        //Yes choice
        findSubsets(str, ans+str.charAt(i), i+1);
        //No choice
        findSubsets(str, ans, i+1);
    }

    // TO find and print all the permutations of a string.
    public static void printPermutations(String str,String ans){
        //base case
        if(str.length()==0){
            System.out.println(ans);
            return;
        }

        //recursion
        for(int i=0;i<str.length();i++){
            char curr = str.charAt(i);
            // "abcde" = "ab" + "de" = "abde"
            String NewStr = str.substring(0, i)+str.substring(i+1); // here ending index means length-1.
            printPermutations(NewStr, ans+curr);
        }
    }
    public static void main(String args[]){
        // int arr[] = new int[5];
        // changeArr(arr,0,1);
        // pritnArr(arr);
        String str = "abc";
        //findSubsets(str,"", 0);
        printPermutations(str,"");
    }
}