LeetCode 236: Lowest Common Ancestor of a Binary Tree

// **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.


/**
 * Definition for a binary tree node.
 * class TreeNode {
 *     val: number
 *     left: TreeNode | null
 *     right: TreeNode | null
 *     constructor(val?: number, left?: TreeNode | null, right?: TreeNode | null) {
 *         this.val = (val===undefined ? 0 : val)
 *         this.left = (left===undefined ? null : left)
 *         this.right = (right===undefined ? null : right)
 *     }
 * }
 */

function lowestCommonAncestor(root: TreeNode | null, p: TreeNode | null, q: TreeNode | null): TreeNode | null {
    if (root === null)
        return null;

    if (root.val === p.val || root.val === q.val)
        return root;

    const lcaInLeft = lowestCommonAncestor(root.left, p, q);
    const lcaInRight = lowestCommonAncestor(root.right, p, q);

    if (lcaInLeft && lcaInRight)
        return root;

    return lcaInLeft ? lcaInLeft : lcaInRight;
}