Binary Tree Inorder Traversal

题目：迭代中序遍历

思路：
1. 递归 ==> 栈
2. 对于每个节点，向左遍历，所有节点入栈，直到叶子节点

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
    public List<Integer> inorderTraversal(TreeNode root) {
        LinkedList<TreeNode> stack = new LinkedList<TreeNode> ();
        List<Integer> list = new LinkedList<Integer> ();
        if(root == null) return list;

        getLeft(stack , root);

        while(!stack.isEmpty()) {
            TreeNode cur = stack.removeLast();
            list.add(cur.val);
            getLeft(stack ,cur.right);
        }

        return list;
    }

    void getLeft(LinkedList<TreeNode> stack , TreeNode root) {
        while(root != null) {
            stack.add(root);
            root = root.left;
        }
    }
}