LeetCode 1650. Lowest Common Ancestor of a Binary Tree III

Question

Given two nodes of a binary tree p and q, return their lowest common ancestor (LCA).

Each node will have a reference to its parent node. The definition for Node is below:

class Node {
    public int val;
    public Node left;
    public Node right;
    public Node parent;
}

According to the definition of LCA on Wikipedia: “The lowest common ancestor of two nodes p and q in a 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).”

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.

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 exist in the tree.

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

Solution

This question is actually a disguised LeetCode 160. Intersection of Two Linked Lists. The lowest common ancestor is the head of intersection list.

public Node lowestCommonAncestor(Node p, Node q) {
    Node first = p;
    Node second = q;
    while (first != second) {
        if (first == null) {
            first = q;
            continue;
        }
        if (second == null) {
            second = p;
            continue;
        }
        first = first.parent;
        second = second.parent;
    }
    return first;
}