_454.java

package com.fishercoder.solutions;

import java.util.HashMap;
import java.util.Map;

/**
 * 454. 4Sum II
 *
 * Given four lists A, B, C, D of integer values,
 * compute how many tuples (i, j, k, l) there are such that A[i] + B[j] + C[k] + D[l] is zero.
 * To make problem a bit easier, all A, B, C, D have same length of N where 0 ≤ N ≤ 500.
 * All integers are in the range of -228 to 228 - 1 and the result is guaranteed to be at most 231 - 1.

 Example:

 Input:
 A = [ 1, 2]
 B = [-2,-1]
 C = [-1, 2]
 D = [ 0, 2]

 Output:
 2

 Explanation:
 The two tuples are:
 1. (0, 0, 0, 1) -> A[0] + B[0] + C[0] + D[1] = 1 + (-2) + (-1) + 2 = 0
 2. (1, 1, 0, 0) -> A[1] + B[1] + C[0] + D[0] = 2 + (-1) + (-1) + 0 = 0*/

public class _454 {

    public static class Solution1 {
        public int fourSumCount(int[] A, int[] B, int[] C, int[] D) {
            Map<Integer, Integer> map = new HashMap();
            int result = 0;
            int len = A.length;
            for (int i = 0; i < len; i++) {
                for (int j = 0; j < len; j++) {
                    int sum = A[i] + B[j];
                    if (map.containsKey(sum)) {
                        map.put(sum, map.get(sum) + 1);
                    } else {
                        map.put(sum, 1);
                    }
                }
            }

            for (int i = 0; i < len; i++) {
                for (int j = 0; j < len; j++) {
                    int sum = -(C[i] + D[j]);
                    if (map.containsKey(sum)) {
                        result += map.get(sum);
                    }
                }
            }

            return result;
        }
    }
}