二叉树的遍历
先序遍历（Preorder Traversal）
根-左-右
1. 递归
Java
1 /** 2 * Definition for a binary tree node. 3 * public class TreeNode { 4 * int val; 5 * TreeNode left; 6 * TreeNode right; 7 * TreeNode(int x) { val = x; } 8 * } 9 */ 10 class Solution { 11 public List<Integer> preorderTraversal(TreeNode root) { 12 List<Integer> result = new ArrayList<Integer>(); 13 if (root != null) { 14 result.add(root.val); 15 result.addAll(preorderTraversal(root.left)); 16 result.addAll(preorderTraversal(root.right)); 17 } 18 return result; 19 } 20 }// 1 ms
Python
1 # Definition for a binary tree node. 2 # class TreeNode(object): 3 # def __init__(self, x): 4 # self.val = x 5 # self.left = None 6 # self.right = None 7 8 class Solution(object): 9 def preorderTraversal(self, root): 10 """ 11 :type root: TreeNode 12 :rtype: List[int] 13 """ 14 if root == None: 15 return [] 16 return [root.val] + self.preorderTraversal(root.left) + self.preorderTraversal(root.right)
2. 栈-迭代（入栈时记录值）
Java
1 /** 2 * Definition for a binary tree node. 3 * public class TreeNode { 4 * int val; 5 * TreeNode left; 6 * TreeNode right; 7 * TreeNode(int x) { val = x; } 8 * } 9 */ 10 class Solution { 11 public List<Integer> preorderTraversal(TreeNode root) { 12 List<Integer> result = new ArrayList<Integer>(); 13 Stack<TreeNode> stack = new Stack<TreeNode>(); 14 while(root != null || !stack.empty()) { 15 if(root != null) { 16 result.add(root.val); 17 stack.add(root); 18 root = root.left; 19 } else { 20 root = stack.pop().right; 21 } 22 } 23 return result; 24 } 25 }// 2 ms
Python
1 # Definition for a binary tree node. 2 # class TreeNode(object): 3 # def __init__(self, x): 4 # self.val = x 5 # self.left = None 6 # self.right = None 7 8 class Solution(object): 9 def preorderTraversal(self, root): 10 """ 11 :type root: TreeNode 12 :rtype: List[int] 13 """ 14 stack, result =[], [] 15 while stack or root: 16 if root: 17 stack.append(root) 18 result.append(root.val) 19 root = root.left 20 else: 21 root = stack.pop().right 22 return result
中序遍历（Inorder Traversal）
左-根-右
1. 递归
Java
1 /** 2 * Definition for a binary tree node. 3 * public class TreeNode { 4 * int val; 5 * TreeNode left; 6 * TreeNode right; 7 * TreeNode(int x) { val = x; } 8 * } 9 */ 10 class Solution { 11 public List<Integer> inorderTraversal(TreeNode root) { 12 List<Integer> result = new ArrayList<Integer>(); 13 if(root != null) { 14 result.addAll(inorderTraversal(root.left)); 15 result.add(root.val); 16 result.addAll(inorderTraversal(root.right)); 17 } 18 return result; 19 } 20 }// 1 ms
Python
1 # Definition for a binary tree node. 2 # class TreeNode(object): 3 # def __init__(self, x): 4 # self.val = x 5 # self.left = None 6 # self.right = None 7 8 class Solution(object): 9 def inorderTraversal(self, root): 10 """ 11 :type root: TreeNode 12 :rtype: List[int] 13 """ 14 if root == None: 15 return [] 16 return self.inorderTraversal(root.left) + [root.val] + self.inorderTraversal(root.right)
2. 栈-迭代（出栈时记录值）
Java
1 /** 2 * Definition for a binary tree node. 3 * public class TreeNode { 4 * int val; 5 * TreeNode left; 6 * TreeNode right; 7 * TreeNode(int x) { val = x; } 8 * } 9 */ 10 class Solution { 11 public List<Integer> inorderTraversal(TreeNode root) { 12 List<Integer> result = new ArrayList<Integer>(); 13 Stack<TreeNode> stack = new Stack<TreeNode>(); 14 TreeNode tmpNode = null; 15 while(root != null || !stack.empty()) { 16 if(root != null) { 17 stack.push(root); 18 root = root.left; 19 } else { 20 tmpNode = stack.pop(); 21 result.add(tmpNode.val); 22 root = tmpNode.right; 23 } 24 } 25 return result; 26 } 27 }// 2 ms
Python
1 # Definition for a binary tree node. 2 # class TreeNode(object): 3 # def __init__(self, x): 4 # self.val = x 5 # self.left = None 6 # self.right = None 7 8 class Solution(object): 9 def inorderTraversal(self, root): 10 """ 11 :type root: TreeNode 12 :rtype: List[int] 13 """ 14 stack, result = [], [] 15 while stack or root: 16 if root: 17 stack.append(root) 18 root = root.left 19 else: 20 root = stack.pop() 21 result.append(root.val) 22 root = root.right 23 return result
