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 (≤) which is the total number of nodes in a tree (and hence the nodes are numbered from 1 to N). Then 2 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
#include <iostream>
#include <vector>
#include <stack>
#include <cstring> using namespace std; vector<int> pre,in,post,val; void postorder(int root,int start,int end){
if(start>end) return ;
int root_index=;
while(root_index<=end&&in[root_index]!=pre[root]) root_index++;
postorder(root+,start,root_index-);
postorder(root++root_index-start,root_index+,end);
post.push_back(pre[root]);
}
int main()
{
stack<int> sta;
int k=,N;
char ch[];int num;
scanf("%d",&N);
while(~scanf("%s",&ch)){
if(strlen(ch)==) {
scanf("%d",&num);
pre.push_back(num);
sta.push(num);
}else{
in.push_back(sta.top());
sta.pop();
if(in.size()==N) break;
}
}
postorder(,,N-);
for(int i=;i<N;i++)
if(i!=N-) printf("%d ",post[i]);
else printf("%d",post[i]); system("pause");
return ;
}