Leetcode: Path Sum

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.

思路分析：

采用二叉树深度遍历即可。

/**
 * Definition for binary tree
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution
{
private:
    bool dfs(TreeNode *node, int sum, int cursum)
    {
        if (node == NULL)
        {
            return false;
        }
    
        if (node->left == NULL && node->right == NULL)
        {
            return node->val + cursum == sum;
        }
    
        return dfs(node->left, sum, cursum + node->val) || dfs(node->right, sum, cursum + node->val);
    }
public:
    bool hasPathSum(TreeNode *root, int sum)
    {
        return dfs(root, sum, 0);
    }
};