Binary Tree Traversal(Preorder, Inorder, Postorder )

Given a binary tree, return the preorder traversal of its nodes’ values.

For example:

Given binary tree {1,#,2,3},

1

\

2

/

3

return [1,2,3].

Note: Recursive solution is trivial, could you do it iteratively?

144. Binary Tree Preorder Traversal

94. Binary Tree Inorder Traversal

145. Binary Tree Postorder Traversal

递归：

//Recursive  preorder  Runtime: 3 ms
/**
* Definition for a binary tree node.
* 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> res;
if(root==NULL){
return res;
}
if(root){
res.push_back(root->val);
}
if(root->left){
vector<int> leftv=preorderTraversal(root->left);
res.insert(res.end(),leftv.begin(),leftv.end());
}
if(root->right){
vector<int> rightv=preorderTraversal(root->right);
res.insert(res.end(),rightv.begin(),rightv.end());
}
return res;
}
};

//Recursive  inorder Runtime: 0 ms
class Solution {
public:
vector<int> inorderTraversal(TreeNode* root) {
vector<int> res;
if(root==NULL){
return res;
}
if(root->left){
vector<int> leftv=inorderTraversal(root->left);
res.insert(res.end(),leftv.begin(),leftv.end());
}
res.push_back(root->val);
if(root->right){
vector<int> rightv=inorderTraversal(root->right);
res.insert(res.end(),rightv.begin(),rightv.end());
}
return res;
}
};

//Recursive  postorder  Runtime: 0 ms
class Solution {
public:
vector<int> postorderTraversal(TreeNode* root) {
vector<int> res;
if(root==NULL){
return res;
}
if(root->left){
vector<int> leftv=postorderTraversal(root->left);
res.insert(res.end(),leftv.begin(),leftv.end());
}
if(root->right){
vector<int> rightv=postorderTraversal(root->right);
res.insert(res.end(),rightv.begin(),rightv.end());
}
res.push_back(root->val);
return res;
}
}

迭代：

//iterative    preorder    Runtime: 0 ms
class Solution {
public:
vector<int> preorderTraversal(TreeNode* root) {
vector<int> res;
stack<TreeNode*> prestack;
while(root||!prestack.empty()){
while(root){
prestack.push(root);
res.push_back(root->val);
root=root->left;
}
root=prestack.top();
prestack.pop();
root=root->right;
}
return res;
}
};

// iterative inorder Runtime: 3 ms
class Solution {
public:
vector<int> inorderTraversal(TreeNode* root) {
vector<int> res;
stack<TreeNode*> prestack;
while(root||!prestack.empty()){
while(root){
prestack.push(root);

root=root->left;
}
root=prestack.top();
res.push_back(root->val);
prestack.pop();
root=root->right;
}
return res;
}
};

//interative postorder Runtime: 0 ms
class Solution {
public:
vector<int> postorderTraversal(TreeNode* root) {
vector<int> res;
stack<TreeNode*> prestack;
while(root||!prestack.empty()){
while(root){
prestack.push(root);
res.insert(res.begin(),root->val);
root=root->right;
}
root=prestack.top();
prestack.pop();
root=root->left;
}
return res;
}
};