UVa 11234 Expressions (二叉树重建&由叶往根的层次遍历)

画图出来后结果很明显

2
xyPzwIM
abcABdefgCDEF

sample output
wzyxIPM
gfCecbDdAaEBF

    *
 +     -
x y  z  w

      F
   B    E
  a A  d D
   b c  e C
         f g

#include<cstdio>
#include<cstring>
#include<iostream>
#include<string>
#include<algorithm>
#include<stack>
#include<queue>
using namespace std;

struct node
{
        node(node* l, node* r, char ch): left(l), right(r), c(ch) {}
        node* left;
        node* right;
        char c;
}*root;

void build_tree(string & str)
{
        stack<node*> st;
        for(int i=0;i<str.length();i++)
        {
                if(islower(str[i]))
                {
                        st.push(new node(0, 0, str[i]));
                }
                else
                {
                        node* right=st.top(); st.pop();
                        node* left=st.top(); st.pop();
                        st.push(new node(left, right, str[i]));
                }
        }

        root=st.top();
}

vector<char> ans;

void bfs()
{
        ans.clear();
        queue<node*> q;
        q.push(root);
        while(!q.empty())
        {
                node* nd=q.front();q.pop();
                ans.push_back(nd->c);
                if(nd->left)
                {
                        q.push(nd->left);
                }

                if(nd->right)
                {
                        q.push(nd->right);
                }
        }
}

void delete_tree(node* nd)
{
        if(nd)
        {
                delete_tree(nd->left);
                delete_tree(nd->right);
                delete nd;
        }
}

void output()
{
        for(int i=ans.size()-1;i>=0;i--)
                cout<<ans[i];
        cout<<endl;
}

int main()
{
        int n;
        cin>>n;
        string str;
        while(n--)
        {
                cin>>str;
                build_tree(str);
                bfs();
                delete_tree(root);
                output();
        }

    return 0;
}