BJOI2018 Summation

Although a preprocessingwaterQuestion, but I don't know what's wrong recently, and the code frequently has bugs. There is actually no case where the subscript is out of bounds to -1.

#include <cstdio>
#include <cstring>
#define N 300010
typedef long long ll;
const ll mo = 998244353;
inline char gc() {
    static char now[1<<16], *S, *T;
    if(S == T) {T = (S = now) + fread(now, 1, 1<<16, stdin); if(S == T) return EOF;}
    return *S++;
}
inline int read() {
    int x = 0, f = 1; char c = gc();
    while(c < '0' || c > '9') {if(c == '-') f = -1; c = gc();}
    while(c >= '0' && c <= '9') {x = x * 10 + c - 48; c = gc();}
    return x * f;
}
int n, m; ll sum[N][55];
inline ll qpow(ll a, ll b, ll c) {
	ll ret = 1;
	while(b) {
		if(b & 1) {--b; ret = ret * a % c;}
		a = a * a % c; b>>= 1;
	}
	return ret;
}
int head[N], cnt = 1; struct adj {int to, next;}e[N<<1];
inline void ins(int x, int y) {e[++cnt].to = y; e[cnt].next = head[x]; head[x] = cnt;}
int d[N], f[N][19];
void dfs(int x, int fa, int dep) {
	f [x] [0] = fa; d [x] = dep;
	for(int i = head[x]; i; i = e[i].next) if(e[i].to != fa) dfs(e[i].to, x, dep + 1);
}
inline void init() {
	dfs(1, 1, 0);
	for(int j = 1; j <= 18; ++j)
		for(int i = 1; i <= n; ++i) f[i][j] = f[f[i][j - 1]][j - 1];
}
inline int lca(int x, int y) {
	if(d[x] < d[y]) {int tmp = x; x = y; y = tmp;}
	for(int i = 18; i >= 0; --i) if(d[x] - d[y] >= (1<<i)) x = f[x][i];
	if(x == y) return x;
	for(int i = 18; i >= 0; --i) if(f[x][i] != f[y][i]) {x = f[x][i]; y = f[y][i];}
	return f[x][0];
}
int main() {
	n = read();
	for(int i = 0; i <= n; ++i)
		for(int j = 0; j <= 50; ++j)
			if(!i) sum[i][j] = 0;
			else sum[i][j] = (sum[i - 1][j] + qpow(i, j, mo)) % mo;
	memset(head, 0, sizeof(head));
	for(int i = 1; i < n; ++i) {int x = read(), y = read(); ins(x, y); ins(y, x);}
	init(); m = read();
	for(int i = 1; i <= m; ++i) {
		int x = read(), y = read(), k = read();
		int z = lca(x, y);
		if(z == 1) {printf("%lld\n", (sum[d[x]][k] + sum[d[y]][k]) % mo); continue;}
		ll ans = ((sum[d[x]][k] - sum[d[z] - 1][k] + sum[d[y]][k] - sum[d[z]][k]) % mo + mo) % mo;
		printf("%lld\n", ans);
	}
	return 0;
}

In addition, it is not very good to use fast power when preprocessing sum... Open an array to memorize the current power of each number, and multiply it directly to update.