LeetCode 1644. Lowest Common Ancestor of a Binary Tree II

Question

Given the root of a binary tree, return the lowest common ancestor (LCA) of two given nodes, p and q. If either node p or q does not exist in the tree, return null. All values of the nodes in the tree are unique.

According to the definition of LCA on Wikipedia: “The lowest common ancestor of two nodes p and q in a binary tree T is the lowest node that has both p and q as descendants (where we allow a node to be a descendant of itself)”. A descendant of a node x is a node y that is on the path from node x to some leaf node.

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

Solution

• Solution1

class Solution {
    public TreeNode lowestCommonAncestor(TreeNode root, TreeNode p, TreeNode q) {
        TreeNode pNode = dfs(root, p.val), qNode = dfs(root, q.val);
        return (qNode == null || pNode == null) ? null : lca(root, pNode, qNode);
    }
    private TreeNode dfs(TreeNode cur, int target){
        if (cur == null) return null;
        if (cur.val == target) return cur;
        TreeNode left = dfs(cur.left, target);
        TreeNode right = dfs(cur.right, target);
        return left == null ? right : left;
    }
    public TreeNode lca(TreeNode root, TreeNode p, TreeNode q) {
        if (root == null || root == p || root == q) return root;
        TreeNode left = lca(root.left, p, q);
        TreeNode right = lca(root.right, p, q);
        if (left != null && right != null) return root;
        if (left != null) return left;
        if (right != null) return right;
        return null;
    }
}

Complexity:

Time complexity: O(n)

Space complexity: O(n)