题目地址:Binary Tree Inorder Traversal - LeetCode
Given a binary tree, return the inorder traversal of its nodes’ values.
Example:
Input: [1,null,2,3]
1
\
2
/
3
Output: [1,3,2]
Follow up: Recursive solution is trivial, could you do it iteratively?
经典的二叉树中序遍历,有迭代和递归2种做法,递归的比较简单。
Python解法如下:
# Definition for a binary tree node.
# class TreeNode:
# def __init__(self, x):
# self.val = x
# self.left = None
# self.right = None
class Solution:
def inorderTraversal(self, root: TreeNode) -> List[int]:
def helper(root, l):
if root is None:
return
helper(root.left,l)
l.append(root.val)
helper(root.right,l)
l = []
if root is None:
return l
helper(root, l)
return l
C++解法如下:
/**
* 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> l;
void helper(TreeNode *root) {
if (root == nullptr) {
return;
}
helper(root->left);
l.push_back(root->val);
helper(root->right);
}
vector<int> inorderTraversal(TreeNode *root) {
l.clear();
helper(root);
return l;
}
};
递归的解法总是比迭代简单,迭代解法如下:
Python解法如下:
# Definition for a binary tree node.
# class TreeNode:
# def __init__(self, x):
# self.val = x
# self.left = None
# self.right = None
class Solution:
def inorderTraversal(self, root: TreeNode) -> List[int]:
if root == None:
return []
l = []
res = []
curr = root
while curr != None or l != []:
while curr is not None:
l.append(curr)
curr = curr.left
curr = l.pop(-1)
res.append(curr.val)
curr = curr.right
return res
时间复杂度为O(n),空间复杂度为O(1)。
C++解法如下:
/**
* 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> inorderTraversal(TreeNode *root) {
vector<int> result;
stack<TreeNode *> l;
if (root == nullptr) {
return result;
}
TreeNode *curr = root;
while (curr != nullptr || !l.empty()) {
while (curr != nullptr) {
l.push(curr);
curr = curr->left;
}
curr = l.top();
l.pop();
result.push_back(curr->val);
curr = curr->right;
}
return result;
}
};