中序遍历

class Solution:
    # 返回对应节点TreeNode
    def KthNode(self, pRoot, k):
        # write code here
        def dfs(root):
            if not root: return 
            dfs(root.left)
            if self.count == 0: return 
            self.count -= 1
            if self.count == 0: 
                self.ans = root
            dfs(root.right)

        if not pRoot or k <= 0: return None
        self.count = k
        self.ans = None
        dfs(pRoot)
        return self.ans