LeetCode 236. Lowest Common Ancestor of a Binary Tree

Question

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).”

https://leetcode.com/problems/lowest-common-ancestor-of-a-binary-tree/

Solution

• Solution1

class Solution {
  public TreeNode lowestCommonAncestor(TreeNode root, TreeNode p, TreeNode q) {    
    if (root == null || root == p || root == q) return root;
    TreeNode l = lowestCommonAncestor(root.left, p, q);
    TreeNode r = lowestCommonAncestor(root.right, p, q);
    if (l == null || r == null) return l == null ? r : l;
    return root;
  }
}

Complexity:

Time complexity: O(n)

Space complexity: O(n)