题目描述

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

方法思路

Approach1: 递归 recursive

class Solution {
    //Runtime: 0 ms, faster than 100.00%
    //Memory Usage: 36.3 MB, less than 14.66% 
    List<Integer> res = new LinkedList<>();
    public List<Integer> preorderTraversal(TreeNode root) {
        if(root == null) return res;
        res.add(root.val);
        preorderTraversal(root.left);
        preorderTraversal(root.right);
        return res;
    }
}

Approach2: iteratively

class Solution{
    //Runtime: 0 ms, faster than 100.00% 
    //Memory Usage: 36.3 MB, less than 11.50%
    public List<Integer> preorderTraversal(TreeNode root) {
        List<Integer> res = new LinkedList<>();
        if(root == null) return res;
        Stack<TreeNode> stack = new Stack<>();
        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;
    }
}