判断是不是二叉搜索树

def solve(tree):
    nodes = list(map(int, tree.split(',')))
    def is_bst(arr, start, end, min_val, max_val):
        if start > end:
            return True
        val = arr[start]
        if val <= min_val or val >= max_val:
            return False
        i = start + 1
        while i <= end and arr[i] < val:
            i += 1
        return is_bst(arr, start + 1, i - 1, min_val, val) and is_bst(arr, i, end, val, max_val)
    print('true' if is_bst(nodes, 0, len(nodes) - 1, float('-inf'), float('inf')) else 'false')