二叉树的层次遍历-Python 两种写法（列表生成式）

1、扫描一遍 一边生成遍历结果一边给deque添加后继节点

def Print(self, pRoot):
        # write code here
        if not pRoot:
            return []
        ans=[]
        deque=[pRoot]
        while deque:
             length=len(deque)
             tmp=[]
             for _ in range(length):
                 curr=deque.pop(0)
                 tmp.append(curr.val)
                 if curr.left:deque.append(curr.left)
                 if curr.right:deque.append(curr.right)
             ans.append(tmp)
        return ans

2、扫描两遍，生成和添加分开处理

def Print(self, pRoot):
        # write code here
        if not pRoot:
            return []
        ans=[]
        deque=[pRoot]
        while deque:
              layer=deque
              ans.append([x.val for x in deque])
              deque=[x for l in layer for x in [l.left,l.right] if x]

        return ans