LeetCode: Path Sum

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.

解題分析：

找路徑問題，找到滿足和爲指定的數的路徑，若找到這樣的路徑，則返回true，否則返回false.

可以使用深度優先搜索的思想進行遞歸求解，若當前結點爲NULL，返回false，若當前結點爲葉子結點，且符合條件，則返回true，否則遞歸判斷其左右子樹。

代碼如下：

bool hasPathSum(TreeNode *root, int sum) {
        if (root == NULL) return false;
        if (root->val == sum && root->left ==  NULL && root->right == NULL) return true;
        return hasPathSum(root->left, sum-root->val) || hasPathSum(root->right, sum-root->val);
    }