236. Lowest Common Ancestor of a Binary Tree

Given a binary tree, find the lowest common ancestor (LCA) of two given nodes in the tree.

According to the definition of LCA on Wikipedia: "The lowest common ancestor is defined between two nodes p and q as the lowest node in T that has both p and q as descendants (where we allow a node to be a descendant of itself )."

Example 1:

Input: root = [3,5,1,6,2,0,8,null,null,7,4], p = 5, q = 1
Output: 3
Explanation: The LCA of nodes 5 and 1 is 3.

Example 2:

Input: root = [3,5,1,6,2,0,8,null,null,7,4], p = 5, q = 4
Output: 5
Explanation: The LCA of nodes 5 and 4 is 5, since a node can be a descendant of itself according to the LCA definition.

Example 3:

Input: root = [1,2], p = 1, q = 2
Output: 1

Constraints:

• The number of nodes in the tree is in the range [2, 105].
• -109 <= Node.val <= 109
• All Node.val are unique.
• p != q
• p and q will exist in the tree.

# @lc code=start
using LeetCode

function lowest_common_ancestor_236(
    root::TreeNode{Int}, p::TreeNode{Int}, q::TreeNode{Int}
)::TreeNode{Int}
    function dfs(node::Union{TreeNode{Int},Nothing})
        (isnothing(node) || node == p || node == q) && return node
        left, right = dfs(node.left), dfs(node.right)
        isnothing(left) && isnothing(right) && return nothing
        !isnothing(left) && !isnothing(right) && return node
        return !isnothing(left) ? left : right
    end
    return dfs(root)
end
# @lc code=end

lowest_common_ancestor_236 (generic function with 1 method)

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