https://oj.leetcode.com/problems/binary-tree-inorder-traversal/

Binary Tree Inorder Traversal.java

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

坑爹的List。。。。初始化哦~