寻找二叉搜索树第k小的节点的值

思路：首先二叉搜索树的是所有右子树大于根节点，所有的左子树小于根节点。

对二叉搜索树进行中序遍历，即从小到大将数放到栈中，当放入的第k个时，就为第k小的元素

/*** 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 {Stack<Integer> stack = new Stack<>();int res;boolean flag = false;public int kthSmallest(TreeNode root, int k) {if(root == null) return 0;kthSmallest(root.left,k);stack.push(root.val);if(stack.size() == k){if(!flag){res = stack.pop();}flag = true;}if(!flag){kthSmallest(root.right,k);}return res;}
}