144. Binary Tree Preorder Traversal

时间：2019-03-15 14:39:42 阅读：125 评论：0 收藏：0 [点我收藏+]

given a binary tree, return the preorder traversal of its nodes‘ values.

Solution 1://非递归

public class Solution {

public List<Integer> preorderTraversal(TreeNode root) {

List<Integer> result = new ArrayList<>();

if (root == null) {

return result;

}

Stack<TreeNode> stack = new Stack<>();

stack.push(root);

while (!stack.empty()) {

TreeNode node = stack.pop();

result.add(node.val);

if (node.right != null) {

stack.push(node.right);

}

if (node.left != null) {

stack.push(node.left);

}

return result;

}

Solution 2：//递归

public class Solution {

List<Integer> result = new ArrayList<>();

public List<Integer> preorderTraversal(TreeNode root) {

if (root != null)

helper(root);

return result;

}

private void helper(TreeNode root) {

result.add(root.val);

if (root.left != null)

helper(root.left);

if (root.right != null)

helper(root.right);

}

原文地址：https://www.cnblogs.com/MarkLeeBYR/p/10536706.html

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