PathSum.swift

/**
 * Question Link: https://leetcode.com/problems/path-sum/
 * Primary idea: recursion
 * Time Complexity: O(n), Space Complexity: O(n)
 *
 * Definition for a binary tree node.
 * public class TreeNode {
 *     public var val: Int
 *     public var left: TreeNode?
 *     public var right: TreeNode?
 *     public init(_ val: Int) {
 *         self.val = val
 *         self.left = nil
 *         self.right = nil
 *     }
 * }
 */

class PathSum {
    func hasPathSum(_ root: TreeNode?, _ sum: Int) -> Bool {
        guard let root = root else {
            return false
        }
        if sum == root.val && root.left == nil && root.right == nil {
            return true
        }
        
        return hasPathSum(root.left, sum - root.val) || hasPathSum(root.right, sum - root.val)
    }
}