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题意
454. 4Sum II
Medium
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: 2Explanation: 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
分析过程及主要思路：
代码实现；
Java：
class Solution {public int fourSumCount(int[] A, int[] B, int[] C, int[] D) {Map<Integer,Integer> map=new HashMap<>();int res=0;for(int i=0;i<A.length;++i){//注意是两层循环不是一层循环for(int j=0;j<B.length;++j){int sumAB=A[i]+B[j];map.put(sumAB,map.getOrDefault(sumAB,0)+1);}}for(int i=0;i<C.length;++i){for(int j=0;j<D.length;++j){int sumCD=C[i]+D[j];res+=map.getOrDefault(-sumCD,0);}}return res;}
}Python：
class Solution:def fourSumCount(self, A: List[int], B: List[int], C: List[int], D: List[int]) -> int:hashtable={}for a in A:for b in B:if a+b in hashtable:hashtable[a+b]+=1else:hashtable[a+b]=1res=0for c in C:for d in D:if -(c+d) in hashtable:res+=hashtable[-(c+d)]return res
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