终于完全会了，我真是太菜了！竟然一直忘了一个很小的地方导致没对.....

一.中序和先序求后序

题目：P1827 美国血统 American Heritage

#include<iostream>
#include<cstdio>
#include<algorithm>
#include<cmath>
#include<queue>
#include<stack>
#include<vector>
#include<map>
#include<string>
#include<cstring>
#define ll long long int
using namespace std;
const int maxn=999999999;
const int minn=-999999999;
inline int read() {
    char c = getchar();
    int x = 0, f = 1;
    while(c < '0' || c > '9') {
        if(c == '-') f = -1;
        c = getchar();
    }
    while(c >= '0' && c <= '9') x = x * 10 + c - '0', c = getchar();
    return x * f;
}
string zhong,qian;
void search(int l1,int r1,int l2,int r2) {
    int place=zhong.find(qian[l2]);
    if(place>l1)    search(l1,place-1,l2+1,l2+(place-l1)+1);
    if(place<r1)    search(place+1,r1,l2+(place-l1)+1,r2);
    cout<<qian[l2];
}
int main() {
    cin>>zhong>>qian;
    int len1=zhong.length();
    int len2=qian.length();
    search(0,len1-1,0,len2-1);
    return 0;
}

二.求先序

题目:P1030 求先序排列

#include<bits/stdc++.h>
using namespace std;

inline int read() {
    char c=getchar();
    int x=0,f=1;
    while(c<'0'||c>'9') {
        if(c=='-') f=-1,c=getchar();
    }
    while(c>='0'&&c<='9') {
        x=x*10+(c-'0') ,c=getchar();
    }
    return x*f;
}
string zhong,hou;
int len1,len2;
void search(int l1,int r1,int l2,int r2)
{
    int place=zhong.find(hou[r2]);
    cout<<zhong[place]; 
    if(place>l1) search(l1,place-1,l2,place-l1+l2-1);//有左子树的情况 
    if(place<r1) search(place+1,r1,place-l1+l2,r2-1);//有右子树的情况
}
int main()
{
    cin>>zhong>>hou;
    len1=zhong.length();
    len2=hou.length();
    search(0,len1-1,0,len2-1);
    return 0;
}