leetcode：Binary Tree Inorder Traversal

二叉树的中序遍历

/**
 * Definition for binary tree
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
    static ArrayList<Integer> ans ; 
    public ArrayList<Integer> inorderTraversal(TreeNode root) {
        ans = new ArrayList<>();
        travel(root);
        return ans;
    }
    
    void travel(TreeNode node){
        if(node != null ){
            travel(node.left);
            ans.add(node.val);
            travel(node.right);
        }
    }
}