LC Tree

简单

100. 相同树

#Method1
def isSameTree(self,p,q):
	if not p and not q:
		return True
	elif not p or not q:
		return False
	else:
		return p.val == q.val and self.isSameTree(p.left,q.left) and self.isSameTree(p.right,q.right)

101. 对称树

def MirrorTree(self,root):
	#Method1 recursion
	if not root:
		return True
	def isMirror(root1,root2):
		if not root1 and not root2:
			return True
		if root1 and root2 and root1.val==root2.val:
			return isMirror(root1.left,root2.right) and isMirror(root1.right,root2.left)
		return False
	return isMirror(root.left,root.right)
	#Method2 iteration
	stack = [root]
        while stack:
            nodes,ans = [],[]
            for x in stack:
                if not x: 
                    ans.append(None)
                    continue
                ans.append(x.val)
                nodes.append(x.left)
                nodes.append(x.right)
            stack = nodes
            if ans!=ans[::-1]:
                return False
        return True

104. 二叉树的最大深度

def maxDepth(self,root):
	if not root:
		return 0
	return 1+max(self.maxDepth(root.left),self.maxDepth(root.right))

107. 二叉树的层序遍历 II

#Method1 iteration
def levelOrderBottom(self, root):
	#Method1 iteration
    stack,res = [root],[]
    while stack:
        nodes,ans= [],[]
        for x in stack:
            if not x: continue
            ans.append(x.val)
            nodes.append(x.left)
            nodes.append(x.right)
        stack = nodes
        res.append(ans)
    return res[::-1][1:]
    #Method2 recursion
    if not root: return []
        res = []
        def dfs(root,depth):
            if not root: return
            if depth == len(res):
                res.append([])
            res[depth].append(root.val)
            dfs(root.left,depth+1)
            dfs(root.right,depth+1)
        dfs(root,0)
        return res[::-1]

110. 平衡二叉树

def isBalanced(self, root):
	if not root: return True
    return abs(self.depth(root.left)-self.depth(root.right))<=1 and self.isBalanced(root.left) and self.isBalanced(root.right)

def depth(self,root):
	if not root:
		return 0
    return 1+max(self.depth(root.left),self.depth(root.right))

111. 二叉树最小深度：

def minDepth(self, root):
	if not root: return 0
    if root.left and root.right:
    	return min(self.minDepth(root.left),self.minDepth(root.right))+1
    elif root.left:
        return 1+self.minDepth(root.left)
    elif root.right:
        return 1+self.minDepth(root.right)
    else:
        return 1

112. 路径总和

def hasPathSum(self, root, sum_):
    res = []
    def dfs(root,tmp):
        if not root: return
        if not root.left and not root.right and sum(tmp)+root.val==sum_:
            res.append(1)
            return 
        dfs(root.left,tmp+[root.val])
        dfs(root.right,tmp+[root.val])
    dfs(root,[])
    return len(res)>0

235. 二叉搜索树的最近公共祖先

def lowestCommonAncestor(self, root, p, q):
	if p.val<root.val and q.val<root.val:
		return self.lowestCommonAncestor(root.left,p,q)
	if p.val > root.val and q.val>root.val:
		return self.lowestCommonAncestor(root.right,p,q)
    return root

257. 二叉树的所有路径

def binaryTreePaths(self, root):
    res = []
    def dfs(root,tmp):
        if not root: return 
        if not root.left and not root.right:
            res.append(tmp+[str(root.val)])
        dfs(root.left,tmp+[str(root.val)])
        dfs(root.right,tmp+[str(root.val)])
    dfs(root,[])
    return ['->'.join(x) for x in res]

543. 二叉树的直径

def diameterOfBinaryTree(self, root):
    self.res = 0
    def dfs(root):
        if not root: return 0
        l,r =dfs(root.left),dfs(root.right)
        self.res = max(self.res,l+r)
        return max(l,r)+1
    dfs(root)
    return self.res

572. 另一个树的子树

def isSubtree(self, s: TreeNode, t: TreeNode) -> bool:
	if not s and not t: return True
    if not s and t or not t and s: return False
    return self.isSametree(s,t) or self.isSubtree(s.left,t) or self.isSubtree(s.right,t)
    
def isSametree(self,x,y):
    if not x and not y:
        return True
    if (not x and y) or (x and not y):
        return False
    else:
        return x.val==y.val and self.isSametree(x.left,y.left) and self.isSametree(x.right,y.right) 

669. 修剪二叉搜索树

def trimBST(self,root,L,R):
    if not root:
        return None
    if root.val<L:
        return self.trimBST(root.right,L,R)
    if root.val>R:
        return self.trimBST(root.left,L,R)
    root.left = self.trimBST(root.left,L,root.val)
    root.right = self.trimBST(root.right,root.val,R)
    return root

589. N叉树的前序遍历

def preorder(self,root):
    #Method1 recursion
    if not root: return []
    res = [root.val]
    for child in root.children:
        res += self.preorder(child)
    return res
    #Method2 iteration
    if not root: return []
    stack,res = [root],[]
    while stack:
        node = stack.pop()
        res.append(node.val)
        stack += node.children[::-1]
    return res

中等

困难