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  • 0454. 4Sum II (M)

    4Sum II (M)

    题目

    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
    

    题意

    给定四个数组,在各个数组中取出一个数使其和为0,求这样的数对的个数。

    思路

    用HashMap记录其中两个数组的组合情况,再遍历另外两个数组的组合进行处理。


    代码实现

    Java

    class Solution {
        public int fourSumCount(int[] A, int[] B, int[] C, int[] D) {
            int count = 0;
            Map<Integer, Integer> hash = new HashMap<>();
            
            for (int i = 0; i < A.length; i++) {
                for (int j = 0; j < B.length; j++) {
                    int sum = A[i] + B[j];
                    hash.put(sum, hash.getOrDefault(sum, 0) + 1);
                }
            }
    
            for (int i = 0; i < C.length; i++) {
                for (int j = 0; j < D.length; j++) {
                    int target = - C[i] - D[j];
                    if (hash.containsKey(target)) {
                        count += hash.get(target);
                    }
                }
            }
            
            return count;
        }
    }
    
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  • 原文地址:https://www.cnblogs.com/mapoos/p/14150426.html
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