How can I make this backtracking solution efficient

general
1

Compute nearest larger number by interchanging digits updated.

Given 2 numbers a and b find the smallest number greater than b by interchanging the digits of a and if not possible print -1.

import java.util.Arrays;

import java.util.Scanner;

class Test {
static int ans = Integer.MAX_VALUE;

static void f(char[] a, String s, int r, boolean[] vis, int b) {
    // System.out.println(s);
    if (r == 0) {
        int x = Integer.parseInt(s);
        if (x > b && x < ans)
            ans = Math.min(x, ans);

        return;
    }

    for (int i = 0; i < a.length; i++) {
        if (!vis[i]) {
            s += a[i];
            vis[i] = true;
            f(a, s, r - 1, vis, b);
            s = s.substring(0, s.length() - 1);
            vis[i] = false;
        }
    }

}

public static void main(String[] args) {
    Scanner sc = new Scanner(System.in);
    char[] a = sc.next().toCharArray();
    // System.out.println(Arrays.toString(a));
    boolean[] vis = new boolean[a.length];
    int b = sc.nextInt();
    f(a, "", a.length, vis, b);

    System.out.println(ans == Integer.MAX_VALUE ? -1 : ans);

}

}

This is the working solution but how Can I make it fast. I will be glad if you give any suggestions.