【LeetCode with Python】 Path Sum
2014-06-27 18:00
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原题页面:https://oj.leetcode.com/problems/path-sum/
题目类型:递归迭代,搜索剪枝
难度评价:★
本文地址:/article/1377522.html
Given a binary tree and a sum, determine if the tree has a root-to-leaf path such that adding up all the values along the path equals the given sum.
For example:
Given the below binary tree and
return true, as there exist a root-to-leaf path
简单的递归算法。但是注意一点,当树为空树时,无论sum是多少都一定要返回False,即使此时sum为0。另外因为只要确定有解即可,因此如果某次迭代发现left已经找到解,那么就不需要再去右子树中寻找解,即及时剪枝。
但是,因为可能有负数出现,所以不能因为path sum已经大于指定sum值就认为该分支可以剪出。
原题页面:https://oj.leetcode.com/problems/path-sum/
题目类型:递归迭代,搜索剪枝
难度评价:★
本文地址:/article/1377522.html
Given a binary tree and a sum, determine if the tree has a root-to-leaf path such that adding up all the values along the path equals the given sum.
For example:
Given the below binary tree and
sum = 22,
5 / \ 4 8 / / \ 11 13 4 / \ \ 7 2 1
return true, as there exist a root-to-leaf path
5->4->11->2which sum is 22.
简单的递归算法。但是注意一点,当树为空树时,无论sum是多少都一定要返回False,即使此时sum为0。另外因为只要确定有解即可,因此如果某次迭代发现left已经找到解,那么就不需要再去右子树中寻找解,即及时剪枝。
但是,因为可能有负数出现,所以不能因为path sum已经大于指定sum值就认为该分支可以剪出。
class Solution: def doHasPathSum(self, root, total, sum): if None == root.left and None == root.right: return sum == (total + root.val) left= False right = False if None != root.left: left = self.doHasPathSum(root.left, total + root.val, sum) if False != left and None != root.right: # pruning right = self.doHasPathSum(root.right, total + root.val, sum) return left or right # @param root, a tree node # @param sum, an integer # @return a boolean def hasPathSum(self, root, sum): if None == root: return False ### return self.doHasPathSum(root, 0, sum)
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