FourSumII.java

package medium;

import java.util.HashMap;
import java.util.Map;
/**
 * ClassName: FourSumII.java
 * Author: chenyiAlone
 * Create Time: 2019/9/17 21:58
 * Description: No.454 4Sum II
 * 思路:
 *
 *      1. 两两一组，使用 HashMap
 *      2. 查表，map 记录一个 key 出现的次数
 *
 * 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 FourSumII {

    public int fourSumCount(int[] A, int[] B, int[] C, int[] D) {
        Map<Integer, Integer> map = new HashMap<>();
        int ret = 0;
        for (int i : A)
            for (int j : B) {
                if (!map.containsKey(i + j)) map.put(i + j, 0);
                map.put(i + j, map.get(i + j) + 1);
            }

        for (int i : C)
            for (int j : D)
                if (map.containsKey(-(i + j))) ret += map.get(-(i + j));
        return ret;
    }
}