[LeetCode] Path Sum

Question：

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.

1、题型分类：

2、思路：采用递归，需要注意的是left和right不能分开递归返回，否则可能出现left不存在但是right不存在，但是left先返回的情况

3、时间复杂度：

4、代码：

public boolean hasPathSum(TreeNode root, int sum) {
if(root!=null) sum=sum-root.val;else return false;
if(root.left==null && root.right==null && sum==0) return true;
if(root.left!=null || root.right!=null)
return hasPathSum(root.left, sum)  || hasPathSum(root.right, sum);
return false;
}

5、优化：

6、扩展：