LintCode-67.二叉树的中序遍历

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/**
 * Definition of TreeNode:
 * public class TreeNode {
 *     public int val;
 *     public TreeNode left, right;
 *     public TreeNode(int val) {
 *         this.val = val;
 *         this.left = this.right = null;
 *     }
 * }
 */
public class Solution {
    /**
     * @param root: The root of binary tree.
     * @return: Inorder in ArrayList which contains node values.
     */
    public ArrayList<Integer> inorderTraversal(TreeNode root) {
        // write your code here
         ArrayList<Integer> a = new ArrayList<Integer>();
        inOrder(a, root);
        return a;
    }
    
    private void inOrder(ArrayList<Integer> a, TreeNode root){
        if(root == null) return;
        inOrder(a, root.left);
        a.add(root.val);
        inOrder(a, root.right);
    }
}