UVa 11234 Expressions

　　先构建一个表达式树，构建方式：从右向左读字符串，如果该字符是大写字母，则构建右子树，接着构建左子树，用递归进行实现。接着层次遍历该树，把得到的结果逆序输出。

　　代码如下：

#include <cstdio>
#include <cstring>
#include <cstdlib>
#include <queue>
#include <cctype>
using namespace std;

const int maxn = 10000+10;

struct Node
{
    char value;
    Node *left, *right;
};

Node * newnode()
{
    Node * u = (Node *)malloc(sizeof(Node));
    if(u != NULL)
    {
        u->value = '\0';
        u->left = u->right = NULL;
    }
    return u;
}

int len, cur;
char s[maxn];

void build(Node * p)
{
    if(cur < 0)   return;
    p->value = s[cur--];
    if(isupper(p->value))
    {
        if(p->right == NULL)   p->right = newnode();
        build(p->right);
        if(p->left == NULL)   p->left = newnode();
        build(p->left);
    }
}
    
void remove_tree(Node * u)
{
    if(u)   return;
    remove_tree(u->left);
    remove_tree(u->right);
    free(u);
}

Node * root;

char ans[maxn];
void traversal(Node * root)
{
    queue<Node *> q;
    q.push(root);
    int n = 0;
    while(!q.empty())
    {
        Node * u = q.front();
        ans[n++] = u->value;
        q.pop();
        if(u->left)   q.push(u->left);
        if(u->right)   q.push(u->right);
    }
}

int main()
{
#ifdef LOCAL    
    freopen("in", "r", stdin);
#endif
    int T;
    scanf("%d", &T);
    while(T--)
    {
        scanf("%s", s);
        len = strlen(s);
        cur = len - 1;
        root = newnode();
        build(root);
        traversal(root);
        remove_tree(root);
        for(int i = len-1; i >= 0; i--)
            printf("%c", ans[i]);
        printf("\n");
    }
    return 0;
}

　　代码中试着用了queue，不小心忘写using namespace std了，编译出错那个纠结啊...