【URAL 1486】Equal Squares

#include<cstdio>
#include<cstring>
#include<algorithm>
using namespace std;
typedef unsigned long long ll;
const int N = 503;
const ll p = 100007;
const int p1 = 1007;
const int p2 = 10007;

struct node {
	ll to; int nxt, x, y;
	node (ll _to = 0, int _nxt = 0, int _x = 0, int _y = 0) : to(_to), nxt(_nxt), x(_x), y(_y) {}
} E[N * N];

char s[N][N];
int point[p + 3], cnt, n, m;
ll hash1[N][N], hash[N][N], pow1[N], pow2[N];


void ins(ll from, ll to, int x, int y) {E[++cnt] = node(to, point[from], x, y); point[from] = cnt;}
int __(ll t) {
	ll f = t % p;
	for(int i = point[f]; i; i = E[i].nxt)
		if (E[i].to == t) return i;
	return 0;
}
bool _(int t) {
	memset(point, 0, sizeof(point)); cnt = 0;
	ll num;
	for(int i = 1; i <= n - t + 1; ++i)
		for(int j = 1; j <= m - t + 1; ++j) {
			num = hash[i + t - 1][j + t - 1] - hash[i + t - 1][j - 1] * pow1[t] - hash[i - 1][j + t - 1] * pow2[t] + hash[i - 1][j - 1] * pow1[t] * pow2[t];
			if (__(num)) return 1;
			ins(num % p, num, i, j);
		}
	return 0;
}

int main() {
	scanf("%d%d", &n, &m);
	for(int i = 1; i <= n; ++i) scanf("%s", s[i] + 1);
	pow1[0] = 1; pow2[0] = 1;
	for(int i = 1; i < N; ++i) pow1[i] = pow1[i - 1] * p1, pow2[i] = pow2[i - 1] * p2;
	for(int i = 1; i <= n; ++i)
		for(int j = 1; j <= m; ++j) {
			hash1[i][j] = hash1[i][j - 1] * p1 + (int) s[i][j];
			hash[i][j] = hash[i - 1][j] * p2 + hash1[i][j];
		}
	int left = 0, right = n == m ? n - 1 : min(n, m), mid;
	while (left < right) {
		mid = (left + right + 1) >> 1;
		if (_(mid)) left = mid;
		else right = mid - 1;
	}
	if (left == 0) puts("0");
	else {
		printf("%d\n", left);
		memset(point, 0, sizeof(point)); cnt = 0;
		for(int i = 1; i <= n - left + 1; ++i)
			for(int j = 1; j <= m - left + 1; ++j) {
			ll num = hash[i + left - 1][j + left - 1] - hash[i + left - 1][j - 1] * pow1[left] - hash[i - 1][j + left - 1] * pow2[left] + hash[i - 1][j - 1] * pow1[left] * pow2[left];
			if (right = __(num)) {
				printf("%d %d\n%d %d\n", E[right].x, E[right].y, i, j);
				return 0;
			}
			ins(num % p, num, i, j);
		}
	}
	
	return 0;
}