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[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

Rust Solution

struct Solution;
use std::collections::HashMap;

impl Solution {
fn four_sum_count(a: Vec<i32>, b: Vec<i32>, c: Vec<i32>, d: Vec<i32>) -> i32 {
let mut hm: HashMap<i32, usize> = HashMap::new();
for &i in &a {
for &j in &b {
*hm.entry(i + j).or_default() += 1;
}
}
let mut res = 0;
for &i in &c {
for &j in &d {
if let Some(v) = hm.get(&(-i - j)) {
res += v;
}
}
}
res as i32
}
}

#[test]
fn test() {
let a = vec![1, 2];
let b = vec![-2, -1];
let c = vec![-1, 2];
let d = vec![0, 2];
let res = 2;
assert_eq!(Solution::four_sum_count(a, b, c, d), res);
}

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