inorder.java

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

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