PAT 甲级 1086. Tree Traversals Again (25)
2017-10-05 18:51
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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:
Sample Output:
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
#include <algorithm> #include <stack> #include <cstring> #include <iostream> #include <vector> using namespace std; vector<int> pre, in, post; void postorder(int root, int start, int end) { if (start > end) return; int i = start; while (i < end && in[i] != pre[root]) i++; postorder(root + 1, start, i - 1); postorder(root + 1 + i - start, i + 1, end); post.push_back(pre[root]); } int main() { int n; scanf("%d", &n); char str[5]; stack<int> s; while (~scanf("%s", str)) { if (strlen(str) == 4) { int num; scanf("%d", &num); pre.push_back(num); s.push(num); } else { in.push_back(s.top()); s.pop(); } } postorder(0, 0, n - 1); printf("%d", post[0]); for (int i = 1; i < n; i++) printf(" %d", post[i]); cin >> n; return 0; }
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