Given a binary tree and a sum, find all root-to-leaf paths where each path’s sum equals the given sum. 
For example: 
Given the below binary tree and sum = 22,

return

这个题目适合用递归来解，我的C++代码实现如下：

class Solution {
public:
    vector<vector<int> > pathSum(TreeNode *root, int sum) {
        vector<vector<int> > paths;
        vector<int> curr_path;
        if (!root) return paths;

        find(root, sum, paths, curr_path);

        return paths;
    }

private:
    void find(TreeNode *root, int sum, vector<vector<int> > &paths, vector<int> &curr_path) {
        curr_path.push_back(root->val);
        if (root->left == nullptr && root->right == nullptr)
            if (root->val == sum) paths.push_back(curr_path);
        if (root->left) {
            find(root->left, sum - root->val, paths, curr_path);
            curr_path.pop_back();
        }
        if (root->right) {
            find(root->right, sum - root->val, paths, curr_path);
            curr_path.pop_back();
        }
    }
};

时间性能如下图所示：