LeetCode [1650] Lowest Common Ancestor of a Binary Tree III

 1650. Lowest Common Ancestor of a Binary Tree III

Medium

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.

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 exist in the tree.

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/* // Definition for a Node. class Node { public int val; public Node left; public Node right; public Node parent; }; */ class Solution { public Node lowestCommonAncestor(Node p, Node q) { Node root = p; while(root.parent!=null) root = root.parent; return helper(root, p, q); } Node helper(Node node, Node p, Node q){ if(node==null) return null; if(node.val == p.val || node.val == q.val) return node; Node l = helper(node.left, p, q); Node r = helper(node.right, p, q); if(l!=null && r!=null) return node; if(l!=null) return l; return r; } }