标签：开始 new blog [] lists bit i++ length 优化
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
一开始想了一个O（n^3）, space O(N)的做法，后来发现还可以优化
Solution： time O(N^2), space O(N^2)
public int fourSumCount(int[] A, int[] B, int[] C, int[] D) {
HashMap<Integer, Integer> map = new HashMap<Integer, Integer>();
for (int i = 0; i < A.length; i++) {
for (int j = 0; j < B.length; j++) {
map.put(A[i] + B[j], map.getOrDefault(A[i] + B[j], 0) + 1);
}
}
int res = 0;
for (int i = 0; i < C.length; i++) {
for (int j = 0; j < D.length; j++) {
res += map.getOrDefault(-(C[i] + D[j]), 0);
}
}
return res;
}
标签：开始 new blog [] lists bit i++ length 优化
原文地址：http://www.cnblogs.com/apanda009/p/7324899.html
