1086 Tree Traversals Again (25分)

An inorder binary tree traversal can be implemented in a non-recursive way with a stack. For example, suppose that when a 6-node binary tree (with the keys numbered from 1 to 6) is traversed, the stack operations are: push(1); push(2); push(3); pop(); pop(); push(4); pop(); pop(); push(5); push(6); pop(); pop(). Then a unique binary tree (shown in Figure 1) can be generated from this sequence of operations. Your task is to give the postorder traversal sequence of this tree.

Figure 1

Input Specification:
Each input file contains one test case. For each case, the first line contains a positive integer N (≤30) which is the total number of nodes in a tree (and hence the nodes are numbered from 1 to N). Then 2N lines follow, each describes a stack operation in the format: “Push X” where X is the index of the node being pushed onto the stack; or “Pop” meaning to pop one node from the stack.

Output Specification:
For each test case, print the postorder traversal sequence of the corresponding tree in one line. A solution is guaranteed to exist. All the numbers must be separated by exactly one space, and there must be no extra space at the end of the line.

Sample Input:

6
Push 1
Push 2
Push 3
Pop
Pop
Push 4
Pop
Pop
Push 5
Push 6
Pop
Pop

Sample Output:

3 4 2 6 5 1

Code:

#include<stdio.h>
#include<string.h>
#include<stack>
#include<algorithm>
using namespace std;
#define maxn 50
struct node{
    int data;
    node* lchild;
    node* rchild;
};
int pre[maxn],in[maxn],post[maxn];
int n;
node* create(int preL,int preR,int inL,int inR){
    if(preL > preR){
        return NULL;
    }
    node* root = new node;
    root->data = pre[preL];
    int i;
    for(i=inL;i<=inR;i++){
        if(pre[preL] == in[i]){
            break;
        }
    }
    int numLeft = i - inL;
    root->lchild = create(preL + 1,preL + numLeft,inL,i-1);
    root->rchild = create(preL + numLeft + 1,preR,i + 1,inR);
    return root;
}
int num = 0;
void postorder(node* root){
    if(root == NULL){
        return;
    }
    postorder(root->lchild);
    postorder(root->rchild);
    printf("%d",root->data);
    num++;
    if(num < n){
        printf(" ");
    }
}
int main(){
    scanf("%d",&n);
    int i;
    char str[5];
    stack<int> st;
    int x,preIndex = 0,inIndex = 0;
    for(i=0;i<2*n;i++){
        scanf("%s",str);
        if(strcmp(str,"Push") == 0){
            scanf("%d",&x);
            pre[preIndex++] = x;
            st.push(x);
        }
        else{
            in[inIndex++] = st.top();
            st.pop();
        }
    }
    node* root = create(0,n-1,0,n-1);
    postorder(root);
    return 0;
}