LintCode - 二叉树的中序遍历（普通）

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难度：容易

要求：

给出一棵二叉树,返回其中序遍历
样例给出一棵二叉树
{1,#,2,3}

 1 
   \ 
    2 
   / 
 3

返回 [1,3,2]

思路：非递归

/**
 * 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 inorderTraversal(TreeNode root) {
        // write your code here
        ArrayList list = new ArrayList();
        Stack stack = new Stack();
        TreeNode node = root;
        while(node != null || !stack.isEmpty()){
            if(node != null){
                stack.push(node);
                node = node.left;
            }else{
                node = stack.pop();
                list.add(node.val);
                node = node.right;
            }
        }
        return list;
    }
}

思路：递归