以下是使用线段树的代码，全测试点RE，但只将addline和dfs两个int函数改成void就AC了。想问下这是什么原理……

ps1:即从int addline(int x, int y) 改为 void addline(int x, int y)

ps2:是否开O2似乎结果一样……而只改dfs函数的时候会134会RE其他TLE，而只改addline函数则全RE

#include <cstdio>
#include <cstring>
#include <stack>
#define DEBUG
#define ll long long
#define MAX 540000
#define swp(x, y) int temp = x; x = y; y = temp
#define rep(i, init, n) for(int i = init; i < n; i++)
#define lowbit(i) ((i)&-(i))
using namespace std;
int nums[MAX*8];
int cnt = 0;
int pos = 1;
int next[MAX*2], to[MAX*2], head[MAX];
int lg[MAX];
int depth[MAX];
int last[MAX];

int query(int x, int y){
    if(x > y){
        swp(x, y);
    }
    int tmin = nums[x];
    if(depth[nums[x]] > depth[nums[y]]){
        tmin = nums[y];
    }
    while((x >> 1)!=(y >> 1)){
        if((x & 1) == 0){
            if(nums[x+1] != -1 && depth[tmin] > depth[nums[x+1]]){
                tmin = nums[x+1];
            }
        }
        if((y & 1) == 1){
            if(nums[y-1] != -1 && depth[tmin] > depth[nums[y-1]]){
                tmin = nums[y-1];
            }
        }
        x >>= 1;
        y >>= 1;
    }

    return tmin;
}

int addline(int x, int y){
    cnt++;
    next[cnt] = head[x];
    to[cnt] = y;
    head[x] = cnt;
}

int dfs(int i, int fa){
    depth[i] = depth[fa]+1;
    nums[pos] = i;
    last[i] = pos;
    pos++;
    for(int p = head[i]; p != 0; p = next[p]){
        if(to[p] == fa){
            continue;
        }
        dfs(to[p], i);
    }
    if(fa != 0){
        nums[pos] = fa;
        last[fa] = pos;
        pos++;
    }
}

int main(){
#ifndef ONLINE_JUDGE
    freopen("1.txt", "r", stdin);
#endif
    int n, m, s;
    scanf("%d %d %d", &n, &m, &s);
    memset(head, 0, sizeof(head));
    memset(nums, -1, sizeof(nums));
    rep(i, 0, n-1){
        int x, y;
        scanf("%d %d", &x, &y);
        addline(x, y);
        addline(y, x);
    }
    depth[0] = 0;
    for(; pos < 2*n; pos <<= 1){
        ;
    }
    int ppos = pos >> 1;
    dfs(s, 0);
    for(; ppos > 0; ppos >>= 1){
        for(int i = ppos; nums[i<<1] != -1 && (i < (ppos << 1)); i++){
            nums[i] = nums[i<<1];
            if(nums[(i<<1)+1] != -1 && depth[nums[i<<1]] > depth[nums[(i<<1)+1]]){
                nums[i] = nums[(i<<1)+1];
            }
        }
    }
    rep(i, 0, m){
        int x, y;
        scanf("%d %d", &x, &y);
        printf("%d\n", query(last[x], last[y]));
    }
    return 0;
}