Lowest Common Ancestor of a Binary Tree

题目介绍

LeetCode 236. Lowest Common Ancestor of a Binary Tree。

复杂度

时间复杂度: O(n), 空间复杂度: O(n)

解题思路

public class TreeNode {
    public var val: Int
    public var left: TreeNode?
    public var right: TreeNode?
    public init(_ val: Int) {
        self.val = val
        self.left = nil
        self.right = nil
    }
}

class Solution {
    private var ans: TreeNode?

    func lowestCommonAncestor(_ root: TreeNode?, _ p: TreeNode?, _ q: TreeNode?) -> TreeNode? {
        dfs(root, p, q)
        return ans
    }

    @discardableResult
    func dfs(_ root: TreeNode?, _ p: TreeNode?, _ q: TreeNode?) -> Bool {
        guard let root = root else {
            return false
        }
        let left = dfs(root.left, p, q)
        let right = dfs(root.right, p, q)
        let mid = root === p || root === q
        if mid ? (left || right) : (left && right) {
            ans = root
        }
        return mid || left || right
    }
}

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