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  • [leedcode 144] 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].

    /**
     * Definition for a binary tree node.
     * public class TreeNode {
     *     int val;
     *     TreeNode left;
     *     TreeNode right;
     *     TreeNode(int x) { val = x; }
     * }
     */
    public class Solution {
        List<Integer> res;
        public List<Integer> preorderTraversal(TreeNode root) {
         /*   //解法一:借助栈数据结构
            List<Integer> res=new ArrayList<Integer>();
            if(root==null) return res;
            Stack<TreeNode> stack=new Stack<TreeNode>();
            TreeNode node=root;
            while(node!=null||!stack.isEmpty()){//条件
                while(node!=null){
                    res.add(node.val);///先序
                    stack.push(node);
                    node=node.left;
    
                }
                node=stack.pop();
                //res.add(node.val);中序
                node=node.right;
            }
            return res;*/
            
           /*解法二:
           res=new ArrayList<Integer>();
            preorder(root);
            return res;
     */
     
        //解法三:此种解法注意push子节点的顺序,先右节点,再左节点
        List<Integer> res=new ArrayList<Integer>();
        Stack<TreeNode> stack=new Stack<TreeNode>();
        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;
        }
        /*解法二:
        public void preorder(TreeNode node){
            if(node==null) return;
            res.add(node.val);
            preorder(node.left);
            preorder(node.right);
        }*/
    }
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  • 原文地址:https://www.cnblogs.com/qiaomu/p/4678962.html
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