def bfs(self, cur, col, ret):
        q = []
        if cur:
            q.append((cur, col))

        while q:
            l = len(q)
            for i in xrange(l):  # avoid non-stop access as in `for elt in q`
                v, c = q[i]
                ret[c].append(v.val)
                if v.left: q.append((v.left, 1-c))
                if v.right: q.append((v.right, c+1))

            q = q[l:]
dataset/ETHPy150Open algorhythms/LeetCode/314 Binary Tree Vertical Order Traversal.py/Solution.bfs/swapped operands of `-`