UVA 1514 - Piece it together（二分图匹配+拆点)

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UVA 1514 - Piece it together

题目链接

题意：给定一个棋盘，上面有黑白或者空，现在有一种L型拼图如图，问能否拼出给定图案，拼图不能重叠

思路：二分图匹配，拆点，对于每个黑点，拆点两个点，一个和横向连，一个和纵向连，然后进行二分图最大匹配，如果能完美匹配就是正确的

代码：

#include <cstdio>#include <cstring>#include <algorithm>using namespace std;const int N = 250005;const int MAXN = 1000005;struct Graph {struct Edge {int u, v, val, next;Edge() {}Edge(int u, int v, int val = 0) {this->u = u;this->v = v;this->val = val;}} edge[MAXN];int first[N], en;void init() {memset(first, -1, sizeof(first));en = 0;}void add(int u, int v, int val = 0) {edge[en] = Edge(u, v, val);edge[en].next = first[u];first[u] = en++;}} g;const int d[4][2] = {{0, -1}, {-1, 0}, {1, 0}, {0, 1}};int t, n, m, b, w, g2[505][505], match[N], vis[N];char str[505][505];bool dfs(int u) {for (int i = g.first[u]; i != -1; i = g.edge[i].next) {int v = g.edge[i].v;if (vis[v]) continue;vis[v] = 1;if (match[v] == -1 || dfs(match[v])) {match[v] = u;return true;}}return false;}bool hungary() {int sum = 0;memset(match, -1, sizeof(match));for (int i = 0; i < 2 * b; i++) {memset(vis, 0, sizeof(vis));if (!dfs(i)) return false;}return true;}int main() {scanf("%d", &t);while (t--) {scanf("%d%d", &n, &m);b = 0; w = 0;g.init();for (int i = 0; i < n; i++) {scanf("%s", str[i]);for (int j = 0; j < m; j++) {if (str[i][j] == 'B')g2[i][j] = b++;else if (str[i][j] == 'W')g2[i][j] = w++;}}if (b * 2 != w) {printf("NO\n");continue;}for (int i = 0; i < n; i++) {for (int j = 0; j < m; j++) {if (str[i][j] == 'B') {for (int k = 0; k < 4; k++) {int x = i + d[k][0];int y = j + d[k][1];if (x < 0 || x >= n || y < 0 || y >= m || str[x][y] != 'W') continue;if (y == j) g.add(g2[i][j] + b, g2[x][y]);else g.add(g2[i][j], g2[x][y]);}}}}if (hungary()) printf("YES\n");else printf("NO\n");}return 0;}

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