代码随想录算法训练营第17天｜二叉树（四）

题目链接：110. 平衡二叉树

思路：后序递归遍历左右子树，判断左右子树是否为平衡树，若左右子树高度差绝对值不超过1，则为平衡树

# Definition for a binary tree node.
# class TreeNode(object):
#     def __init__(self, val=0, left=None, right=None):
#         self.val = val
#         self.left = left
#         self.right = right
class Solution(object):
    def getHeight(self,node):
        if not node:
            return 0
        leftHeight=self.getHeight(node.left)
        if leftHeight==-1:return -1
        rightHeight=self.getHeight(node.right)
        if rightHeight==-1:return -1
 
        if  abs(leftHeight-rightHeight)>1:
            return -1
        else:
            return max(leftHeight,rightHeight)+1
    def isBalanced(self, root):
        """
        :type root: TreeNode
        :rtype: bool
        """
        if self.getHeight(root)==-1:
            return False
        else:
            return True

题目链接：257. 二叉树的所有路径

思路：递归加回溯。

# Definition for a binary tree node.
# class TreeNode(object):
#     def __init__(self, val=0, left=None, right=None):
#         self.val = val
#         self.left = left
#         self.right = right
class Solution(object):
    def travelback(self,cur,path,result):
        path.append(cur.val)
        if cur.left is None and cur.right is None:
            spath='->'.join(map(str,path))
            result.append(spath)
            return 
        if cur.left:
            self.travelback(cur.left,path,result)
            path.pop()
        if cur.right:
            self.travelback(cur.right,path,result)
            path.pop()
 
        
    def binaryTreePaths(self, root):
        """
        :type root: TreeNode
        :rtype: List[str]
        """
        path=[]
        result=[]
        if not root:
            return result
        self.travelback(root,path,result)
        return result
        

题目链接：404. 左叶子之和

思路：左叶子之和，左子树左叶子之和加右子树左叶子之和

# Definition for a binary tree node.
# class TreeNode(object):
#     def __init__(self, val=0, left=None, right=None):
#         self.val = val
#         self.left = left
#         self.right = right
class Solution(object):
    def sumOfLeftLeaves(self, root):
        """
        :type root: TreeNode
        :rtype: int
        """
        if root is None:
            return 0
        if root.left is None and root.right is None:
            return 0
        leftValue=self.sumOfLeftLeaves(root.left)
        if root.left is not None and root.left.left is None and root.left.right is None:
            leftValue=root.left.val
        rightVaue=self.sumOfLeftLeaves(root.right)
        return leftValue+rightVaue