*270 Closest Binary Search Tree Value

https://leetcode.com/problems/closest-binary-search-tree-value/

solution

class Solution:
    def closestValue(self, root, target):
        upper = root
        lower = root

        while root:
            if root.val > target:
                upper = root  # 先记录再往后迭代
                root = root.left
            elif root.val < target:
                lower = root
                root = root.right
            else:
                return root.val

        if abs(upper.val - target) > abs(lower.val - target):
            return lower.val
        return upper.val

# 一些BST题目甚至可以先中序存为list, 再从list中找到最近的
class Solution(object):
    def closestValue(self, root, target):
        res = None
        cur = root

        while cur:
            # 更直接的思路: 记录遍历过程中距离, 并记录更新距离最小的dian
            if res is None or abs(res - target) > abs(cur.val - target):
                res = cur.val

            if cur.val > target:
                cur = cur.left
            else:
                cur = cur.right
        return res

时间复杂度：O(h) 空间复杂度：O(h)

dfs

class Solution:
    def __init__(self):
        self.low = -float('inf')
        self.high = float('inf')

    def closest_value(self, root: TreeNode, target: float) -> int:
        if not root:
            return

        if root.val <= target:
            self.low = max(self.low, root.val)
            self.closest_value(root.right, target)
        else:
            self.high = min(self.high, root.val)
            self.closest_value(root.left, target)

        if target - self.low > self.high -target:
            return self.high
        else:
            return self.low

follow up

272. Closest Binary Search Tree Value II