CF356A Knight Tournament求链表为什么会T

#include<bits/stdc++.h>
#define ri register int
using namespace std;
const int N=3e5+10;

int n,m;
int nex[N],pre[N],ans[N];

int main(){
    scanf("%d%d",&n,&m);
    for(ri i=1;i<=n+1;i++)  nex[i]=i+1,pre[i]=i-1;
    nex[0]=1,pre[0]=0;
    while(m--){
        int l,r,w;
        scanf("%d%d%d",&l,&r,&w);
        for(ri i=l;i<=r&&i<=n;i=nex[i]){
            if(ans[i])  continue;
            if(i!=w){
                ans[i]=w;
                pre[nex[i]]=pre[i];
                nex[pre[i]]=nex[i];
            }
        }
    }
    for(ri i=1;i<=n;i++)    printf("%d ",ans[i]);
}