Solution.ts

/**
 * 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 {
    const find = (root: TreeNode | null) => {
        if (root == null || root == p || root == q) {
            return root;
        }
        const left = find(root.left);
        const right = find(root.right);
        if (left != null && right != null) {
            return root;
        }
        if (left != null) {
            return left;
        }
        return right;
    };
    return find(root);
}