中序遍历

1. 定义一个栈，用于存取二叉树中的元素

Deque<TreeNode> stack = new ArrayDeque<TreeNode>();

2. 进入while循环while(! stack.isEmpty()|| root != null){}
3. 将root的左节点入栈，直到root==null

while(root==null){
    stack.push(root);
    root= root.left;
}

4. 将栈中的节点取出来，--k；

root = stack.pop();
--k;
if(k==0){
break;

5. 开始取右节点

root = root.right;

6. 最后返回root.val

整体代码：

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode() {}
 *     TreeNode(int val) { this.val = val; }
 *     TreeNode(int val, TreeNode left, TreeNode right) {
 *         this.val = val;
 *         this.left = left;
 *         this.right = right;
 *     }
 * }
 */
class Solution {
    public int kthSmallest(TreeNode root, int k) {
       Deque<TreeNode> stack  = new ArrayDeque<TreeNode>();
        while(! stack.isEmpty()||root!=null){
            while(root!=null){
                stack.push(root);
                root = root.left;
            }

            root = stack.pop();
            --k;
            if(k==0){
                break;
            }

            root = root.right;
        }

        return root.val;
    }
}