LeetCode 671 - 二叉树中第二小的节点

class Solution:
    def findSecondMinimumValue(self, root: Optional[TreeNode]) -> int:
        if not root:
            return -1
        # 根节点就是最小值，所以本题主要改动的是secondMin
        firstMin, secondMin = root.val, sys.maxsize

        def findSecondMinimumValueIMPL(root: Optional[TreeNode]):
            nonlocal firstMin, secondMin
            if not root:
                return
            # python3可以这么写判断：仅在这时候需要更新次小值
            if firstMin < root.val < secondMin:
                secondMin = root.val
            # 如果相等才继续遍历，否则不必遍历了，因为剩下的子节点都是比根secondMin大的
            elif root.val == firstMin:
                findSecondMinimumValueIMPL(root.left)
                findSecondMinimumValueIMPL(root.right)

        findSecondMinimumValueIMPL(root)
        if secondMin >= sys.maxsize:
            return -1
        return secondMin