1. 树的前序遍历
/**
* Definition for a binary tree node.
* struct TreeNode {
* int val;
* TreeNode *left;
* TreeNode *right;
* TreeNode(int x) : val(x), left(NULL), right(NULL) {}
* };
*/
// recursion method (递归方法)
class Solution {
public:
vector<int> res;
void recursion(TreeNode* node) {
res.push_back(node->val);
if(node->left recursion(node->left);
if(node->right) recursion(node->right);
}
vector<int> preorderTraversal(TreeNode* root) {
if(!root) return res;
recursion(root);
return res;
}
};
class Solution {
public:
vector<int> preorderTraversal(TreeNode* root) {
vector<int> res;
stack<TreeNode*> s;
if(!root) return res;
while(root || !s.empty()) {
if(root) {
res.push_back(root->val);
s.push(root);
root=root->left;
}
else {
root=s.top()->right;
s.pop();
}
}
return res;
}
};
2. 树的中序遍历
/*** Definition for a binary tree node.
* struct TreeNode {
* int val;
* TreeNode *left;
* TreeNode *right;
* TreeNode(int x) : val(x), left(NULL), right(NULL) {}
* };
*/
// recursion method
class Solution {
public:
vector<int> res;
void recursion(TreeNode* root) {
if(root->left) recursion(root->left);
res.push_back(root->val);
if(root->right) recursion(root->right);
}
vector<int> inorderTraversal(TreeNode* root) {
if(!root) return res;
recursion(root);
return res;
}
};
// not recursion method(非递归方法,迭代,用栈)
class Solution {
public:
vector<int> inorderTraversal(TreeNode* root) {
vector<int> res;
stack<TreeNode*> s;
if(!root) return res;
while(root || !s.empty()) {
if(root) {
s.push(root);
root=root->left;
}
else {
res.push_back(s.top()->val);
root=s.top()->right;
s.pop();
}
}
return res;
}
};
3. 树的后序遍历
/*** Definition for a binary tree node.
* struct TreeNode {
* int val;
* TreeNode *left;
* TreeNode *right;
* TreeNode(int x) : val(x), left(NULL), right(NULL) {}
* };
*/
// recursion method
class Solution {
public:
vector<int> res;
void recursion(TreeNode* node) {
if(node->left) recursion(node->left);
if(node->right) recursion(node->right);
res.push_back(node->val);
}
vector<int> postorderTraversal(TreeNode* root) {
if(!root) return res;
recursion(root);
return res;
}
};
// not recursion method(非递归方法,迭代,用栈。由于是左-右-根的方式,相当于根-右-左的结果去反,因此可以利用类似先序遍历的方法,左右反过来,最后在整体翻转序列。)
class Solution {
public:
vector<int> postorderTraversal(TreeNode* root) {
vector<int> res;
stack<TreeNode*> s;
if(!root) return res;
while(root || !s.empty()) {
if(root) {
res.insert(res.begin(), root->val); // Store the result sequence in a reverse order.
s.push(root);
root=root->right;
}
else {
root=s.top()->left;
s.pop();
}
}
return res;
}
};