这道题，先用kruskal求一遍图中的最大生成树。
然后，倍增求lca，求lca的同时求出边权的最小值。
#include <cstring> #include <cstdio> #include <algorithm> int n, m, cnt, q, t, k; int f[10001], head[100001], p[10001][21], minn[10001][21], deep[10001]; bool vis[10001]; struct node { int x, y, z; }tree[100001]; struct Node { int next, to, val; }edge[100001]; inline void add(int a, int b, int c) { edge[cnt].val = c; edge[cnt].to = b; edge[cnt].next = head[a]; head[a] = cnt++; } inline int father(int a) { return a == f[a] ? a : f[a] = father(f[a]); } inline bool cmp(node a, node b) { return a.z > b.z; } void kruskal() { int i; for(i = 1; i <= m; i++) scanf("%d %d %d", &tree[i].x, &tree[i].y, &tree[i].z); for(i = 1; i <= n; i++) f[i] = i; std::sort(tree + 1, tree + m + 1, cmp); for(i = 1; i <= m; i++) { int fa = father(tree[i].x), fb = father(tree[i].y); if(fa != fb) { f[fa] = fb; add(tree[i].x, tree[i].y, tree[i].z); add(tree[i].y, tree[i].x, tree[i].z); } if(k == n - 1) break; } } void dfs(int i) { int j; vis[i] = 1; for(j = head[i]; j != -1; j = edge[j].next) if(!vis[edge[j].to]) { deep[edge[j].to] = deep[i] + 1; p[edge[j].to][0] = i; minn[edge[j].to][0] = edge[j].val; dfs(edge[j].to); } } void init() { int i, j; for(j = 1; (1 << j) <= n; j++) for(i = 1; i <= n; i++) { p[i][j] = p[p[i][j - 1]][j - 1]; minn[i][j] = std::min(minn[i][j - 1], minn[p[i][j - 1]][j - 1]); } } int lca(int a, int b) { int i, j, ret = 707406378; if(deep[a] < deep[b]) std::swap(a, b); for(i = 0; (1 << i) <= deep[a]; i++); i--; for(j = i; j >= 0; j--) if(deep[a] - (1 << j) >= deep[b]) { ret = std::min(ret, minn[a][j]); a = p[a][j]; } if(a == b) return ret; for(j = i; j >= 0; j--) if(p[a][j] != p[b][j]) { ret = std::min(ret, std::min(minn[a][j], minn[b][j])); a = p[a][j]; b = p[b][j]; } ret = std::min(ret, std::min(minn[a][0], minn[b][0])); return ret; } int main() { int i, j, x1, y1; scanf("%d %d", &n, &m); memset(head, -1, sizeof(head)); //memset(minn, 127 / 3, sizeof(minn)); kruskal(); for(i = 1; i <= n; i++) if(!vis[i]) { deep[i] = 1; dfs(i); } init(); scanf("%d", &q); for(i = 1; i <= q; i++) { scanf("%d %d", &x1, &y1); if(father(x1) != father(y1)) printf("-1 "); else printf("%d ", lca(x1, y1)); } return 0; }
换了写法
惨啊，
i >= 0 我居然nc的用 i 代替
应该是 i >= 1 用 i 代替
1 #include <cstdio> 2 #include <cstring> 3 #include <algorithm> 4 5 const int MAXN = 10001, MAXM = 50001, INF = 707406378; 6 int n, m, q, cnt, tot; 7 int head[MAXM], to[MAXM << 1], next[MAXM << 1], val[MAXM << 1]; 8 int f1[MAXN], f[MAXN][21], min[MAXN][21], deep[MAXN]; 9 10 struct node 11 { 12 int x, y, z; 13 }p[MAXM]; 14 15 inline bool cmp(node a, node b) 16 { 17 return a.z > b.z; 18 } 19 20 inline void add(int x, int y, int z) 21 { 22 to[cnt] = y; 23 val[cnt] = z; 24 next[cnt] = head[x]; 25 head[x] = cnt++; 26 } 27 28 inline int find(int x) 29 { 30 return x == f1[x] ? x : f1[x] = find(f1[x]); 31 } 32 33 inline int Min(int x, int y) 34 { 35 return x < y ? x : y; 36 } 37 38 inline void swap(int &x, int &y) 39 { 40 x ^= y ^= x ^= y; 41 } 42 43 inline void dfs(int u) 44 { 45 int i, v; 46 deep[u] = deep[f[u][0]] + 1; 47 for(i = 0; f[u][i]; i++) 48 f[u][i + 1] = f[f[u][i]][i], 49 min[u][i + 1] = Min(min[u][i], min[f[u][i]][i]); 50 for(i = head[u]; i ^ -1; i = next[i]) 51 { 52 v = to[i]; 53 if(!deep[v]) 54 { 55 f[v][0] = u; 56 min[v][0] = val[i]; 57 dfs(v); 58 } 59 } 60 } 61 62 inline int lca(int x, int y) 63 { 64 int i, ans = INF; 65 if(deep[x] < deep[y]) swap(x, y); 66 for(i = 20; i >= 0; i--) 67 if(deep[f[x][i]] >= deep[y]) 68 ans = Min(ans, min[x][i]), x = f[x][i]; 69 if(x == y) return ans == INF ? -1 : ans; 70 for(i = 20; i >= 0; i--) 71 if(f[x][i] ^ f[y][i]) 72 ans = Min(ans, min[x][i]), 73 ans = Min(ans, min[y][i]), 74 x = f[x][i], y = f[y][i]; 75 ans = Min(ans, min[x][0]); 76 ans = Min(ans, min[y][0]); 77 return ans == INF ? -1 : ans; 78 } 79 80 int main() 81 { 82 //freopen("truck.in", "r", stdin); 83 //freopen("truck.out", "w", stdout); 84 int i, x, y, fx, fy; 85 scanf("%d %d", &n, &m); 86 memset(head, -1, sizeof(head)); 87 memset(min, 127 / 3, sizeof(min)); 88 for(i = 1; i <= m; i++) scanf("%d %d %d", &p[i].x, &p[i].y, &p[i].z); 89 std::sort(p + 1, p + m + 1, cmp); 90 for(i = 1; i <= n; i++) f1[i] = i; 91 for(i = 1; i <= m; i++) 92 { 93 fx = find(p[i].x); 94 fy = find(p[i].y); 95 if(fx ^ fy) 96 { 97 f1[fx] = fy; 98 tot++; 99 add(p[i].x, p[i].y, p[i].z); 100 add(p[i].y, p[i].x, p[i].z); 101 } 102 if(tot == n - 1) break; 103 } 104 for(i = 1; i <= n; i++) 105 if(!deep[i]) 106 dfs(i); 107 scanf("%d", &q); 108 for(i = 1; i <= q; i++) 109 { 110 scanf("%d %d", &x, &y); 111 if(find(x) ^ find(y)) puts("-1"); 112 else printf("%d ", lca(x, y)); 113 } 114 return 0; 115 }