leetcode 144 二叉树前序遍历

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

Example:

Input: [1,null,2,3]
   1
    \
     2
    /
   3

Output: [1,2,3]

递归：

import java.util.ArrayList;
class Solution {
    public List<Integer> preorderTraversal(TreeNode root) {
        ArrayList<Integer> ret=new ArrayList<Integer>();
        if(root!=null){
             ret.add(root.val);
             ret.addAll(preorderTraversal(root.left));
             ret.addAll(preorderTraversal(root.right));
        }
        return ret;
    }
}

迭代：

import java.util.ArrayList;
import java.util.Stack;
class Solution {
    public List<Integer> preorderTraversal(TreeNode root) {
        ArrayList<Integer> ret=new ArrayList<>();
        if(root==null) return ret;
        Stack<TreeNode> s=new Stack<TreeNode>();
        while(root!=null||!s.isEmpty()){
            while(root!=null){
                ret.add(root.val);
                s.push(root);
                root=root.left;
            }
            if(!s.isEmpty()){
                root=s.pop();
                root=root.right;
            }
        }
        return ret;
    }
}