103 Binary Tree Zigzag Level Order Traversal

103 Binary Tree Zigzag Level Order Traversal

level traversal的变种

class Solution:
    # @param {TreeNode} root
    # @return {integer[][]}
    def zigzagLevelOrder(self, root):
        if root == None:
            return []
        ans, q, level, flag = [], [root, None], [], 0
        while True:
            node = q.pop(0)
            if node != None:
                level.append(node.val)
                if node.left != None:
                    q.append(node.left)
                if node.right != None:
                    q.append(node.right)
            else:
                if flag:
                    ans.append(list(reversed(level)))
                else:
                    ans.append(level)
                level = []
                flag ^= 1
                if q == []:
                    break
                else:
                    q.append(None)
        return ans