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leetcode(112):Path Sum

程序员文章站 2024-01-05 11:19:40
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题目: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->2 which sum is 22.

题目分析:判断是否从根结点到叶子节点有和为固定值的分支,需要不断的统计此刻的和,然后对其来进行判断。

python代码实现:

class Solution(object):
    def hasPathSum(self, root, sum):
        """
        :type root: TreeNode
        :type sum: int
        :rtype: bool
        """
        if root == None:
            return False
        if root.left == None and root.right == None:
            if root.val == sum:
                return True
            return False
        if root.left != None:
            root.left.val += root.val
            if self.hasPathSum(root.left, sum) == True:
                return True
        if root.right != None:
            root.right.val += root.val
            if self.hasPathSum(root.right, sum) == True:
                return True
        return False

以上的代码,虽然实现了功能,但不好之处在于改变了每个节点的值,也就是改变了原始的结构,这对于数据来说是毁灭性的求解,所以,肯定是不好的,然后对之进行改进。

# Definition for a binary tree node.
# class TreeNode(object):
#     def __init__(self, x):
#         self.val = x
#         self.left = None
#         self.right = None

class Solution(object):
    def hasPathSum(self, root, sum):
        """
        :type root: TreeNode
        :type sum: int
        :rtype: bool
        """
        if root == None:
            return False
        if root.left == None and root.right == None:
            if root.val == sum:
                return True
        return self.hasPathSum(root.left, sum-root.val) or self.hasPathSum(root.right, sum-root.val)

这样子改进,然后就会好很多了,这样子可以不改变原有的结构,然后还判断出了是否存在这样的一条从根节点到叶子节点的路径和为固定值的判断。

大佬的代码:

class Solution:
    # @param root, a tree node
    # @param sum, an integer
    # @return a boolean
    # 1:27
    def hasPathSum(self, root, sum):
        if not root:
            return False

        if not root.left and not root.right and root.val == sum:
            return True

        sum -= root.val

        return self.hasPathSum(root.left, sum) or self.hasPathSum(root.right, sum)

大佬的递归的思想跟我的思想重合的,只是大佬对于判断条件的写法更加简略,更加简洁,学习就好。

DFS Recursively

def hasPathSum1(self, root, sum):
    res = []
    self.dfs(root, sum, res)
    return any(res)

def dfs(self, root, target, res):
    if root:
        if not root.left and not root.right:
            if root.val == target:
                res.append(True)
        if root.left:
            self.dfs(root.left, target-root.val, res)
        if root.right:
            self.dfs(root.right, target-root.val, res)

这个题目中,我们又学到了any(),内嵌函数的用法。

DFS with stack

def hasPathSum2(self, root, sum):
    if not root:
        return False
    stack = [(root, root.val)]
    while stack:
        curr, val = stack.pop()
        if not curr.left and not curr.right:
            if val == sum:
                return True
        if curr.right:
            stack.append((curr.right, val+curr.right.val))
        if curr.left:
            stack.append((curr.left, val+curr.left.val))
    return False

BFS with queue

def hasPathSum(self, root, sum):
    if not root:
        return False
    queue = [(root, sum-root.val)]
    while queue:
        curr, val = queue.pop(0)
        if not curr.left and not curr.right:
            if val == 0:
                return True
        if curr.left:
            queue.append((curr.left, val-curr.left.val))
        if curr.right:
            queue.append((curr.right, val-curr.right.val))
    return False
相关标签: leetcode

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