257. Binary Tree Paths

原题链接：https://leetcode.com/problems/binary-tree-paths/

两种做法，iterative or recursive. 遍历的做法更快一些也更简单。

def binaryTreePaths(self, root: TreeNode) -> List[str]:
        self.out = []
        if not root:
            return self.out 
        
        level = [(root, str(root.val))]
        
        while level:
            node, path = level.pop()
            if not node.left and not node.right:
                self.out.append(path)
            if node.left:
                level.append((node.left, path+'->'+str(node.left.val)))
            if node.right:
                level.append((node.right, path+'->'+str(node.right.val)))
                            
        return self.out