二叉树的遍历

先序遍历（Preorder Traversal）

根-左-右

1. 递归

Java

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    public List<Integer> preorderTraversal(TreeNode root) {
        List<Integer> result = new ArrayList<Integer>();
        if (root != null) {
            result.add(root.val);
            result.addAll(preorderTraversal(root.left));
            result.addAll(preorderTraversal(root.right));
        }
        return result;
    }
}// 1 ms

Python

# Definition for a binary tree node.
# class TreeNode(object):
#     def __init__(self, x):
#         self.val = x
#         self.left = None
#         self.right = None

class Solution(object):
    def preorderTraversal(self, root):
        """
        :type root: TreeNode
        :rtype: List[int]
        """
        if root == None:
            return []
        return [root.val] + self.preorderTraversal(root.left) + self.preorderTraversal(root.right) 

2. 栈-迭代（入栈时记录值）

 Java

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    public List<Integer> preorderTraversal(TreeNode root) {
        List<Integer> result = new ArrayList<Integer>();
        Stack<TreeNode> stack = new Stack<TreeNode>();
        while(root != null || !stack.empty()) {
            if(root != null) {
                result.add(root.val);
                stack.add(root);
                root = root.left;
            } else {
                root = stack.pop().right;
            }
        }
        return result;
    }
}// 2 ms

Python

# Definition for a binary tree node.
# class TreeNode(object):
#     def __init__(self, x):
#         self.val = x
#         self.left = None
#         self.right = None

class Solution(object):
    def preorderTraversal(self, root):
        """
        :type root: TreeNode
        :rtype: List[int]
        """
        stack, result =[], []
        while stack or root:
            if root:
                stack.append(root)
                result.append(root.val)
                root = root.left
            else:
                root = stack.pop().right
        return result

中序遍历（Inorder Traversal）

左-根-右

1. 递归

Java

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    public List<Integer> inorderTraversal(TreeNode root) {
        List<Integer> result = new ArrayList<Integer>();
        if(root != null) {
            result.addAll(inorderTraversal(root.left));
            result.add(root.val);
            result.addAll(inorderTraversal(root.right));
        }
        return result;
    }
}// 1 ms

Python

# Definition for a binary tree node.
# class TreeNode(object):
#     def __init__(self, x):
#         self.val = x
#         self.left = None
#         self.right = None

class Solution(object):
    def inorderTraversal(self, root):
        """
        :type root: TreeNode
        :rtype: List[int]
        """
        if root == None:
            return []
        return self.inorderTraversal(root.left) + [root.val] + self.inorderTraversal(root.right)

2. 栈-迭代（出栈时记录值）

Java

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    public List<Integer> inorderTraversal(TreeNode root) {
        List<Integer> result = new ArrayList<Integer>();
        Stack<TreeNode> stack = new Stack<TreeNode>();
        TreeNode tmpNode = null;
        while(root != null || !stack.empty()) {
            if(root != null) {
                stack.push(root);
                root = root.left;
            } else {
                tmpNode = stack.pop();
                result.add(tmpNode.val);
                root = tmpNode.right;
            }
        }
        return result;
    }
}// 2 ms

Python

# Definition for a binary tree node.
# class TreeNode(object):
#     def __init__(self, x):
#         self.val = x
#         self.left = None
#         self.right = None

class Solution(object):
    def inorderTraversal(self, root):
        """
        :type root: TreeNode
        :rtype: List[int]
        """
        stack, result = [], []
        while stack or root:
            if root:
                stack.append(root)
                root = root.left
            else:
                root = stack.pop()
                result.append(root.val)
                root = root.right
        return result