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题目：

题解：

class Solution {public int fourSumCount(int[] A, int[] B, int[] C, int[] D) {Map<Integer, Integer> countAB = new HashMap<Integer, Integer>();for (int u : A) {for (int v : B) {countAB.put(u + v, countAB.getOrDefault(u + v, 0) + 1);}}int ans = 0;for (int u : C) {for (int v : D) {if (countAB.containsKey(-u - v)) {ans += countAB.get(-u - v);}}}return ans;}
}