Binary Tree Preorder Traversal python

二叉樹的前序遍歷（非遞歸）（小米雲計算一面）

java

/**
 * Definition of TreeNode:
 * public class TreeNode {
 *     public int val;
 *     public TreeNode left, right;
 *     public TreeNode(int val) {
 *         this.val = val;
 *         this.left = this.right = null;
 *     }
 * }
 */


public class Solution {
    /*
     * @param root: A Tree
     * @return: Preorder in ArrayList which contains node values.
     */
    public List<Integer> preorderTraversal(TreeNode root) {
        // write your code here
        Stack<TreeNode> stack = new Stack<>();
        List<Integer> res = new ArrayList<>();
        if (root == null) {
            return res;
        }
        stack.push(root);
        while (!stack.isEmpty()) {
            TreeNode node = stack.pop();
            res.add(node.val);
            if (node.right != null) {
                stack.push(node.right);
            }
            if (node.left != null) {
                stack.push(node.left);
            }
        }
        return res;
    }
}

python

"""
Definition of TreeNode:
class TreeNode:
    def __init__(self, val):
        self.val = val
        self.left, self.right = None, None
"""


class Solution:
    """
    @param: root: A Tree
    @return: Preorder in ArrayList which contains node values.
    """
    def preorderTraversal(self, root):
        # write your code here
        if root is None:
            return []
        stack, res = [root], []
        while stack:
            node = stack.pop()
            res.append(node.val)
            if node.right is not None:
                stack.append(node.right)
            if node.left is not None:
                stack.append(node.left)
        return res