UVa 11234 - Expressions

#include <iostream>
#include <string>
#include <cctype>
#include <queue>
#include <stack>
using namespace std;

struct Node
{
char c;
Node *left, *right;
Node(char v)
{
left = right = 0;
c = v;
}
};

stack<char> ans;
queue<Node *> scan;
string str;
int i;

Node *Build();
void BFS(Node *root);
void Output();

int main()
{
//freopen("input.txt", "r", stdin);
int n;
cin >> n;
while (n--)
{
cin >> str;
i = str.size() - 1;
//建树.
Node *root = Build();
//层次遍历输出.
scan.push(root);  //先把根节点放入队列.
BFS(root);
Output();
}
return 0;
}

Node *Build()
{
Node *root = new Node(str[i]);
if (islower(str[i]))    //如果当前值是小写字母, 说明没有左右子树了.
{
i--;
return root;
}
i--;
root->right = Build();
root->left = Build();
return root;
}

void BFS(Node *root)
{
Node *now;
while (!scan.empty())
{
now = scan.front();
if (scan.front()->left)	//如果当前结点有左子树. 因为有左子树必有右子树.
{
ans.push(now->left->c);		//接下来两个结点进栈.
ans.push(now->right->c);
scan.push(now->left);	//放队列里
scan.push(now->right);
scan.pop();	//当前结点出队.
}
else
scan.pop();		//不然的话当前结点就是小写字母, 直接出队.
}
}

void Output()
{
while (!ans.empty())
{
cout << ans.top();
ans.pop();
}
cout << str[str.size() - 1];
cout << endl;
}