src/main/java/com/fishercoder/solutions/_1601.java

package com.fishercoder.solutions;

public class _1601 {
    public static class Solution1 {
        int max = 0;

        public int maximumRequests(int n, int[][] requests) {
            helper(requests, 0, new int[n], 0);
            return max;
        }

        private void helper(int[][] requests, int index, int[] count, int num) {
            // Traverse all n buildings to see if they are all 0. (means balanced)
            if (index == requests.length) {
                for (int i : count) {
                    if (0 != i) {
                        return;
                    }
                }
                max = Math.max(max, num);
                return;
            }
            // Choose this request
            count[requests[index][0]]++;
            count[requests[index][1]]--;
            helper(requests, index + 1, count, num + 1);
            count[requests[index][0]]--;
            count[requests[index][1]]++;

            // Not Choose the request
            helper(requests, index + 1, count, num);
        }
    }
}