http://poj.org/problem?id=2785
The SUM problem can be formulated as follows: given four lists A, B, C, D of integer values, compute how many quadruplet (a, b, c, d ) ∈ A x B x C x D are such that a + b + c + d = 0 . In the following, we assume that all lists have the same size n .
The first line of the input file contains the size of the lists n (this value can be as large as 4000). We then have n lines containing four integer values (with absolute value as large as 228 ) that belong respectively to A, B, C and D .
For each input file, your program has to write the number quadruplets whose sum is zero.
6
-45 22 42 -16
-41 -27 56 30
-36 53 -37 77
-36 30 -75 -46
26 -38 -10 62
-32 -54 -6 45
4
0 0 0 0
0 0 0 0
0 0 0 0
0 0 0 0
5
-56 45 34 78
-34 -67 45 -23
-12 -34 -56 46
45 34 -32 8
-23 -56 4 53
5
256
11
哈希大法。。
#include<algorithm>
#include<iostream>
#include<cstdlib>
#include<cstring>
#include<cstdio>
#include<vector>
#include<queue>
#include<set>
using std::abs;
using std::sort;
using std::pair;
using std::swap;
using std::queue;
using std::multiset;
#define pb(e) push_back(e)
#define sz(c) (int)(c).size()
#define mp(a, b) make_pair(a, b)
#define all(c) (c).begin(), (c).end()
#define iter(c) decltype((c).begin())
#define cls(arr, val) memset(arr, val, sizeof(arr))
#define cpresent(c, e) (find(all(c), (e)) != (c).end())
#define rep(i, n) for(int i = 0; i < (int)n; i++)
#define tr(c, i) for(iter(c) i = (c).begin(); i != (c).end(); ++i)
const int N = 5010;
const int M = 3000007;
const int INF = 0x3f3f3f3f;
typedef long long ll;
ll A[4][N];
struct HashSet {
struct Node {
ll v;
int cnt;
}arr[N * N];
int tot, head[M], next[M];
inline void init() {
tot = 0;
rep(i, M) head[i] = -1;
}
inline void insert(ll val) {
bool f = false;
int u = abs(val) % M;
for (int i = head[u]; ~i; i = next[i]) {
if (arr[i].v == val) {
f = true;
arr[i].cnt++;
}
}
if (!f) {
arr[tot].v = val, arr[tot].cnt = 1; next[tot] = head[u], head[u] = tot++;
}
}
inline int find(ll val) {
int u = abs(val) % M;
for (int i = head[u]; ~i; i = next[i]) {
if (arr[i].v == val) return arr[i].cnt;
}
return 0;
}
}work;
int main() {
#ifdef LOCAL
freopen("in.txt", "r", stdin);
freopen("out.txt", "w+", stdout);
#endif
int n;
while (~scanf("%d", &n)) {
ll ans = 0;
work.init();
rep(i, n) {
rep(j, 4) scanf("%lld", &A[j][i]);
}
rep(i, n) {
rep(j, n) {
work.insert(A[0][i] + A[1][j]);
}
}
rep(i, n) {
rep(j, n) {
ans += work.find(-(A[2][i] + A[3][j]));
}
}
printf("%lld\n", ans);
}
return 0;
}
poj 2785 4 Values whose Sum is 0
原文：http://www.cnblogs.com/GadyPu/p/4868382.html