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：
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1 #include <cstdio> 2 #include <cstring> 3 #include <cstdlib> 4 #include <queue> 5 #include <cctype> 6 using namespace std; 7 8 const int maxn = 10000+10; 9 10 struct Node 11 { 12 char value; 13 Node *left, *right; 14 }; 15 16 Node * newnode() 17 { 18 Node * u = (Node *)malloc(sizeof(Node)); 19 if(u != NULL) 20 { 21 u->value = '\0'; 22 u->left = u->right = NULL; 23 } 24 return u; 25 } 26 27 int len, cur; 28 char s[maxn]; 29 30 void build(Node * p) 31 { 32 if(cur < 0) return; 33 p->value = s[cur--]; 34 if(isupper(p->value)) 35 { 36 if(p->right == NULL) p->right = newnode(); 37 build(p->right); 38 if(p->left == NULL) p->left = newnode(); 39 build(p->left); 40 } 41 } 42 43 void remove_tree(Node * u) 44 { 45 if(u) return; 46 remove_tree(u->left); 47 remove_tree(u->right); 48 free(u); 49 } 50 51 Node * root; 52 53 char ans[maxn]; 54 void traversal(Node * root) 55 { 56 queue<Node *> q; 57 q.push(root); 58 int n = 0; 59 while(!q.empty()) 60 { 61 Node * u = q.front(); 62 ans[n++] = u->value; 63 q.pop(); 64 if(u->left) q.push(u->left); 65 if(u->right) q.push(u->right); 66 } 67 } 68 69 int main() 70 { 71 #ifdef LOCAL 72 freopen("in", "r", stdin); 73 #endif 74 int T; 75 scanf("%d", &T); 76 while(T--) 77 { 78 scanf("%s", s); 79 len = strlen(s); 80 cur = len - 1; 81 root = newnode(); 82 build(root); 83 traversal(root); 84 remove_tree(root); 85 for(int i = len-1; i >= 0; i--) 86 printf("%c", ans[i]); 87 printf("\n"); 88 } 89 return 0; 90 }
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...