[leetcode]Binary Tree Postorder TraversalBinary Tree Preorder Traversal

题意：给出了一颗二叉树，求二叉树的前序遍历和后序遍历，题中要求尽量使用非递归方法遍历

后序遍历递归代码：

/*** Definition for binary tree* struct TreeNode {*     int val;*     TreeNode *left;*     TreeNode *right;*     TreeNode(int x) : val(x), left(NULL), right(NULL) {}* };*/
class Solution {
public:void dfs(vector<int> &vec,TreeNode *root){if(root->left!=NULL) dfs(vec,root->left);if(root->right!=NULL) dfs(vec,root->right);vec.push_back(root->val);}vector<int> postorderTraversal(TreeNode *root){vector<int> vec;if(root!=NULL) dfs(vec,root);return vec;}
};

非递归代码：

/*** Definition for binary tree* struct TreeNode {*     int val;*     TreeNode *left;*     TreeNode *right;*     TreeNode(int x) : val(x), left(NULL), right(NULL) {}* };*/
class Solution {
public:vector<int> postorderTraversal(TreeNode *root){vector<int> vec;stack<int> num;stack<TreeNode*> st;if(root!=NULL) st.push(root);TreeNode *node;while(!st.empty()){node=st.top();st.pop();if(node->left!=NULL) st.push(node->left);if(node->right!=NULL) st.push(node->right);num.push(node->val);}while(!num.empty()){vec.push_back(num.top());num.pop();}return vec;}
};

先序遍历递归代码：

/*** Definition for binary tree* struct TreeNode {*     int val;*     TreeNode *left;*     TreeNode *right;*     TreeNode(int x) : val(x), left(NULL), right(NULL) {}* };*/
class Solution {
public:void dfs(vector<int> &vec,TreeNode *root){vec.push_back(root->val);if(root->left!=NULL) dfs(vec,root->left);if(root->right!=NULL) dfs(vec,root->right);}vector<int> preorderTraversal(TreeNode *root){vector<int> vec;if(root!=NULL) dfs(vec,root);return vec;}
};

先序遍历非递归代码：

/*** Definition for binary tree* struct TreeNode {*     int val;*     TreeNode *left;*     TreeNode *right;*     TreeNode(int x) : val(x), left(NULL), right(NULL) {}* };*/
class Solution {
public:vector<int> preorderTraversal(TreeNode *root){vector<int> vec;stack<TreeNode*> st;TreeNode *node;while(root||!st.empty()){while(root){st.push(root);vec.push_back(root->val);root=root->left;}root=st.top();st.pop();root=root->right;}return vec;}
};

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