URAL 1018 Binary Apple Tree（树形dp）

刚开始贪心每次最小的叶子去掉，wa了几次发现思路是错了。正解是简单的树形dp。

#pragma warning(disable:4996)
#include 
#include 
#include 
#include 
using namespace std;
vectorg[105];
int num[105][105], cnt[105];//cnt[u]表示结点u的子树中有几条树枝
int dp[105][105];//dp[u][i]表示结点u的子树中保留i条树枝可获得的最大值
int n, q;
bool vis[105];

void getcnt(int u){
	vis[u] = true;
	cnt[u] = 0;
	for (int i = 0; i < g[u].size(); i++){
		int v = g[u][i];
		if (!vis[v]){
			cnt[u]++;
			getcnt(v);
			cnt[u] += cnt[v];
		}
	}
}

void dfs(int u){
	vis[u] = true;
	int first = 0, second = 0;
	for (int i = 0; i < g[u].size(); i++){
		int v = g[u][i];
		if (!vis[v]){
			if (first)second = v;
			else first = v;
			dfs(v);
		}
	}

	if (second){//2 child
		for (int i = 1; i < n; i++){
			if (i>cnt[u]){
				dp[u][i] = -1;
				continue;
			}
			dp[u][i] = 0;
			if (cnt[first] >= i - 1){
				dp[u][i] = max(dp[u][i], num[u][first] + dp[first][i - 1]);
			}
			if (cnt[second] >= i - 1){
				dp[u][i] = max(dp[u][i], num[u][second] + dp[second][i - 1]);
			}
			//dp[u][i] = max(dp[first][i - 1] + num[u][first], dp[second][i - 1] + num[u][second]);
			int tmp = num[u][first] + num[u][second];
			for (int j = 0; j <= i - 2; j++){
				if (cnt[first] >= j&&cnt[second] >= i - 2 - j)
					dp[u][i] = max(dp[u][i], tmp + dp[first][j] + dp[second][i - 2 - j]);
			}
		}
	}
	else if (first){//1 child
		for (int i = 1; i < n; i++){
			if (i>cnt[u]){
				dp[u][i] = -1;
				continue;
			}

			if (cnt[first] >= i - 1){
				dp[u][i] = num[u][first] + dp[first][i - 1];
			}
			//dp[u][i] = num[u][first] + dp[first][i - 1];
		}
	}
	else{//0 child
		for (int i = 1; i < n; i++)dp[u][i] = -1;
	}
}

int main(){
	//freopen("in.txt", "r", stdin);
	scanf("%d %d", &n, &q);
	for (int i = 1; i < n; i++){
		int u, v, c; scanf("%d %d %d", &u, &v, &c);
		g[u].push_back(v);
		g[v].push_back(u);
		num[u][v] = num[v][u] = c;
	}
	memset(vis, false, sizeof vis);
	getcnt(1);
	memset(vis, false, sizeof vis);
	dfs(1);
	printf("%d\n", dp[1][q]);
	return 0;
}