#include<iostream>
#include<stack>
#include<queue>
using namespace std;/*
二叉树遍历算法递归+非递归：
前序遍历：根->左->右
中序遍历：左->根->右
后序遍历：左->右->根
层次遍历
*/
struct TreeNode {int val;TreeNode *left;TreeNode *right;TreeNode(int x): val(x),left(NULL),right(NULL) {}
};/*递归版本*/
void prerecusive(TreeNode *root)
{if (!root) return;cout << root->val << " ";prerecusive(root->left);prerecusive(root->right);
}
void inrecusive(TreeNode *root)
{if (!root) return;inrecusive(root->left);cout << root->val << " ";inrecusive(root->right);
}
void postrecusive(TreeNode *root)
{if (!root) return;postrecusive(root->left);postrecusive(root->right);cout << root->val << " ";
}/*循环版本。栈做辅助*/
void preiteration(TreeNode *root)
{if (!root) return;stack<TreeNode *> s;s.push(root);while (!s.empty()){　　　　　TreeNode *curr=s.top();cout << curr->val << " ";s.pop();if (curr->right) s.push(curr->right);if (curr->left) s.push(curr->left);}
}
void initeration(TreeNode *root)
{if (!root) return;stack<TreeNode *> s;TreeNode *curr = root;while (curr || !s.empty())//s没有值，第一个判断条件是curr不是空
    {if (curr){s.push(curr);curr = curr->left;}else{cout << s.top()->val << " ";curr = s.top()->right;s.pop();}}
}
void postiteration(TreeNode *root)
{if (!root) return;stack<TreeNode *> s;s.push(root);TreeNode *curr;TreeNode *visited;//记录子节点已经访问过while (!s.empty()){curr = s.top();/** 出栈条件：* 对于叶子节点：直接弹出* 对于非叶子节点：如果已经遍历过其左子节点或右子节点，则弹出*/if ((!curr->left && !curr->right) || (visited && (curr->left==visited || curr->right == visited))){cout << curr->val << " ";visited = curr;s.pop();}else{if (curr->right) s.push(curr->right);if (curr->left) s.push(curr->left);}}
}void leveltraverse(TreeNode *root)
{if (!root) return;queue<TreeNode *> q;TreeNode *curr;q.push(root);while (!q.empty()){curr = q.front();cout << curr->val << " ";q.pop();if (curr->left) q.push(curr->left);if (curr->right) q.push(curr->right);}
}