0112.Path Sum.swift

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     public var val: Int
 *     public var left: TreeNode?
 *     public var right: TreeNode?
 *     public init() { self.val = 0; self.left = nil; self.right = nil; }
 *     public init(_ val: Int) { self.val = val; self.left = nil; self.right = nil; }
 *     public init(_ val: Int, _ left: TreeNode?, _ right: TreeNode?) {
 *         self.val = val
 *         self.left = left
 *         self.right = right
 *     }
 * }
 */
class Solution {
    func hasPathSum(_ root: TreeNode?, _ targetSum: Int) -> Bool {
        guard let root = root else {
            return false
        }
        
        // leaf node 
        if root.left == nil && root.right == nil {
            return targetSum == root.val
        }
        
        return hasPathSum(root.right, targetSum - root.val) || hasPathSum(root.left, targetSum - root.val)
    }
}