Leet-code144. Binary Tree Preorder Traversal

这是一道将二叉树先序遍历，题目不难。

首先采用深搜递归

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
   public static List<Integer> resultlist = new ArrayList<Integer>();

    public static void dfs (TreeNode root ,boolean flag ) {
        if(flag==true)
        {
            resultlist.clear();
        }
        if(root==null)
        {
            return ;
        }

            resultlist.add(root.val);

        if(root.left!=null)
        {
            dfs(root.left,false);
        }
        if(root.right!=null)
        {
            dfs(root.right,false);
        }
        
    }


    public static List<Integer> preorderTraversal(TreeNode root ) {
        dfs(root,true);
        return resultlist;
    }

}

 非递归实现方式

 public static List<Integer> preorderTraversal(TreeNode root ) {
        List<Integer> result = new ArrayList<>();

        if(root == null)
            return result;

        Stack<TreeNode> stack = new Stack<TreeNode>();
        stack.push(root);

        while(!stack.empty()){
            TreeNode n = stack.pop();
            result.add(n.val);

            if(n.right != null){
                stack.push(n.right);
            }
            if(n.left != null){
                stack.push(n.left);
            }

        }
        return result;
    }