leetcode 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 + B + C + D 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 + B + C + D = 1 + (-2) + (-1) + 2 = 0
2. (1, 1, 0, 0) -> A + B + C + D = 2 + (-1) + (-1) + 0 = 0
class Solution {
public int fourSumCount(int[] A, int[] B, int[] C, int[] D) {
HashMap<Integer,Integer> map = new HashMap<>();
for(int i : A){
for(int j: B){
map.merge(i+j,1,(new BiFunction<Integer,Integer,Integer>(){public Integer apply(Integer i1, Integer i2){return i1+i2;}}));
}
}
int ret = 0;
for(int i : C){
for(int j: D){
ret += map.getOrDefault(-i-j,0);
}
}
return ret;
}
}
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