题目链接：https://leetcode.com/problems/binary-tree-inorder-traversal/
题目：
inorder
For example:
Given binary tree {1,#,2,3},
1 \ 2 / 3
[1,3,2].
Note:
思路：
用栈模拟中序遍历过程
算法：
public List<Integer> inorderTraversal(TreeNode root) {
List<Integer> list = new ArrayList<Integer>();
Stack<TreeNode> stack = new Stack<TreeNode>();
while (root != null || !stack.isEmpty()) {
if (root != null) {
stack.push(root);
root = root.left;
} else { //左子树为空，则访问val，且开始遍历右子树
root = stack.pop();
list.add(root.val);
root = root.right;
}
}
return list;
}