複雜度 O(n^2)。記得要檢查 Array Boundary。
Complexity O(n^2). Remember to check array boundaries.

UVa Link

#include <cstdio>
#include <algorithm>

int in[5000];

int main() {
    int n;
    while (scanf("%d", &n) != -1) {
        for (int i = 0; i < n; i++) scanf("%d", &in[i]);
        std::sort(in, in + n);
        unsigned long long ans = 0;
        for (int xi = 0; xi < n - 2; xi++) {
            int yi = xi + 1, zi = xi + 2;
            while (yi < n - 1 && zi < n) {
                if (in[xi] + in[yi] > in[zi]) zi++;
                else if (in[xi] + in[yi] < in[zi]) yi++;
                else {
                    int ycnt = 1, zcnt = 1;
                    while (in[xi] + in[++yi] == in[zi] && yi < n - 1) ycnt++;
                    while (in[xi] + in[yi-1] == in[++zi] && zi < n) zcnt++;
                    ans += ycnt * zcnt;
                }
            }
        }
        printf("%llu\n", ans);
    }
}