二叉树的深度搜索专题一＞二叉搜索树中第 K 小的元素

class Solution {
    private int count;
    private int ret;

    public int kthSmallest(TreeNode root, int k) {
        count = k;
        dfs(root);
        return ret; 
    }

    private void dfs(TreeNode root){
        /**中序遍历+两个全局变量+剪枝
         */

        //剪枝
        if(root == null || count == 0) return;
        dfs(root.left);
        count--;
        if(count == 0) ret = root.val;

        //剪枝
        if(count == 0) return;
        dfs(root.right);
    }
}