LeetCode 144. Binary Tree Preorder Traversal

LC address: https://leetcode.com/problems/binary-tree-preorder-traversal/

Given a binary tree, return the preorder traversal of its nodes’ values.

For example:
Given binary tree {1,#,2,3},

   1
    \
     2
    /
   3

return [1,2,3].

Analysis:

基本Stack的应用（或者用recursion）。

Solution:

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

Solution code can also be found here: https://github.com/all4win/LeetCode.git