LeetCode - 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
这道题借鉴了2sum的解法, 创建一个hashmap, 先遍历A,B所有的组合把相反的和和出现次数存进去, 在遍历C,D的所有组合,找匹配。
class Solution { public int fourSumCount(int[] A, int[] B, int[] C, int[] D) { Map<Integer, Integer> map =new HashMap<>(); int count = 0; for(int i = 0; i < A.length; i++){ for(int j = 0; j< B.length; j++){ int sum = A[i] + B[j]; map.put(0-sum, map.getOrDefault(0-sum,0)+1); } } for(int i = 0; i<C.length; i++){ for(int j = 0; j < D.length; j ++){ int sum = C[i] + D[j]; if(map.containsKey(sum)){ count += map.get(sum); } } } return count; } }
posted on 2018-09-28 11:35 IncredibleThings 阅读(89) 评论(0) 编辑 收藏 举报