Path Sum

Root to leaf sum.

I:

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

II:

If we use cur.pop_back() at the end of the function, then we do not need to backup the current vector.

class Solution {
public:
    void dfs(TreeNode *root, int sum, vector<int> &cur, vector<vector<int> > &res) {
        if (!root) return;
        cur.push_back(root->val);
        vector<int> backup(cur);
        if ((!root->left) && (!root->right) && root->val==sum) {
            res.push_back(cur);
            return;
        }
        dfs(root->left, sum-root->val, cur, res);
        cur=backup;
        dfs(root->right, sum-root->val, cur, res);
    }
    vector<vector<int> > pathSum(TreeNode *root, int sum) {
        vector<int> cur;
        vector<vector<int> > res;
        dfs(root, sum, cur, res);
        return res;
    }
};