Leetcode笔记（回溯基本功）

回溯 17 - 39
现在开始按大类做题，参照 花花酱Huahua 整理的分类 集合 去刷题

今天这两道题都是以前做过的。。。

17. 电话号码的字母组合 - medium

//13.48
//14.01
class Solution {
    vector<string> ans;
    vector<string> letter;
    void helper(int i, string &digits, string str) {
        if(i == digits.size()) {
            //cout << str << endl;
            ans.push_back(str);
            return ;
        }

        int num = digits[i]-'0';
        for(int k = 0; k < letter[num].size(); k++) {
            str = str + letter[num][k];
            helper(i+1, digits, str);
            str.pop_back();
        }
    }  
public:
    vector<string> letterCombinations(string digits) {
        if(digits.empty()) return {};

        letter.resize(10);
        letter[2] = "abc";
        letter[3] = "def";
        letter[4] = "ghi";
        letter[5] = "jkl";
        letter[6] = "mno";
        letter[7] = "pqrs";
        letter[8] = "tuv";
        letter[9] = "wxyz";

        helper(0, digits, "");
        return ans;
    }
};

39. 组合总和 - medium

//15.15
//15.27
class Solution {
    vector<vector<int>> ans;
    void helper(vector<int> &candidates, int target, vector<int> tmp, int k) {
        if(target == 0) {
            ans.push_back(tmp);
            return ;
        }
        if(target < 0) return ;

        for(int i = k; i < candidates.size(); i++) {
            tmp.push_back(candidates[i]);
            //target -= candidates[i];
            helper(candidates, target-candidates[i], tmp, k);
            tmp.pop_back();
            k++;
        }
    }
public:
    vector<vector<int>> combinationSum(vector<int>& candidates, int target) {
        int len = candidates.size();
        if(len == 0) return {};

        //sort(candidates.begin(), candidates.end());
        helper(candidates, target, {}, 0);
        return ans;
    }
};