LeetCode algorithm binary tree—144. Preorder traversal of binary tree

Table of contents

144. Preorder traversal of binary trees - LeetCode

Code:

operation result: 

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Example 1:

Input: root = [1,null,2,3]
 Output: [1,2,3]

Example 2:

Input: root = []
 Output: []

Example 3:

Input: root = [1]
 Output: [1]

Example 4:

Input: root = [1,2]
 Output: [1,2]

Example 5:

Input: root = [1,null,2]
 Output: [1,2]

hint:

• The number of nodes in the tree is  [0, 100] within the range
• -100 <= Node.val <= 100

Advanced: The recursive algorithm is simple, can you do it with an iterative algorithm?

Code:

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode() {}
 *     TreeNode(int val) { this.val = val; }
 *     TreeNode(int val, TreeNode left, TreeNode right) {
 *         this.val = val;
 *         this.left = left;
 *         this.right = right;
 *     }
 * }
 */
class Solution {
    List<Integer> List=new ArrayList<>();
    public List<Integer> preorderTraversal(TreeNode root) {
        fun(root);
        return List;
    }
    public void fun(TreeNode root){
        if(root==null) return;
        List.add(root.val);
        fun(root.left);
        fun(root.right);
    }
}

operation result: