UVA 1484 - Alice and Bob's Trip（树形DP）

#include <stdio.h>
#include <string.h>
#include <vector>
using namespace std;
#define max(a,b) ((a)>(b)?(a):(b))
#define min(a,b) ((a)<(b)?(a):(b))
#define INF 0x3f3f3f3f
const int N = 500005;
int n, l, r, dp[N], E, first[N], next[N];
struct Edge {
	int u, v, w;
} edge[N];

inline void scanf_(int &num)//无负数
{
    char in;
    while((in=getchar()) > ‘9‘ || in<‘0‘) ;
    num=in-‘0‘;
    while(in=getchar(),in>=‘0‘&&in<=‘9‘)
        num*=10,num+=in-‘0‘;
}

void dfs(int u, int fa, int sum, int who) {
	if (who && first[u] != -1) dp[u] = INF;
	else dp[u] = 0;
	for (int i = first[u]; i != -1; i = next[i]) {
		int v = edge[i].v, w = edge[i].w;
		if (v == fa) continue;
		dfs(v, u, sum + w, 1 - who);
		if (who == 0 && dp[v] + w + sum >= l && dp[v] + w + sum <= r)
			dp[u] = max(dp[u], dp[v] + w);
		if (who == 1 && dp[v] + w + sum >= l && dp[v] + w + sum <= r)
			dp[u] = min(dp[u], dp[v] + w);
	}
}

void add(int u, int v, int w) {
	edge[E].u = u; edge[E].v = v; edge[E].w = w;
	next[E] = first[u];
	first[u] = E++;
}

int main() {
	while (~scanf("%d%d%d", &n, &l, &r)) {
		E = 0;
		memset(first, -1, sizeof(first));
		int u, v, w;
		for (int i = 0; i < n - 1; i++) {
			scanf_(u); scanf_(v); scanf_(w);
			add(u, v, w);
		}
		dfs(0, -1, 0, 0);
		if (dp[0] < l || dp[0] > r) printf("Oh, my god!\n");
		else printf("%d\n", dp[0]);

	}
	return 0;
}