先序遍历加中序遍历确定后序遍历

#define bug(x) cout<<#x<<" is "<<x<<endl
#define IO std::ios::sync_with_stdio(0)
#include <bits/stdc++.h>
using namespace  std;
#define ll long long
#define pb push_back
const ll mod=1e9+7;
const int N=2e5+10;

char s[N],t[N];

struct node{
    int c;
    node *lson,*rson;
};

int a[200],b[200],id[200],d[200],n,cnt;

node *root;

node* build(node *now){
    now =new node();
    //now=(node *)malloc(sizeof(node));
    //now->lson=now->rson=NULL;
    int k=id[s[cnt]];  
    d[k]=1;
    now->c=s[cnt++];
    
    if(k>1&&!d[k-1]){
        now->lson=build(now->lson);
    }
    if(k<n&&!d[k+1]){
        now->rson=build(now->rson);
    }
    return now;
}

void preorder(node *now){
    if(now->lson!=NULL)preorder(now->lson);
    if(now->rson!=NULL)preorder(now->rson);
    printf("%c",now->c);
}

void work(){
    preorder(build(root));
}

int main(){
    scanf("%s",s+1);
    scanf("%s",t+1);
    n=strlen(s+1);

    for(int i=1;i<=n;i++){
        id[t[i]]=i;
    }
    work();
}
/*
ABCD
BADC
BDCA

1 2 4 5 3 6
4 2 5 1 6 3
4 5 2 6 3 1
*/