【LC刷题】DAY19：77 216 17

文章目录

【LC刷题】DAY19：77 216 17
- 77. 组合 [link](https://leetcode.cn/problems/combinations/description/)
- 216. 组合总和 III [link](https://leetcode.cn/problems/combination-sum-iii/submissions/542307400/)
- 17. 电话号码的字母组合[link](https://leetcode.cn/problems/letter-combinations-of-a-phone-number/description/)

77. 组合 link

class Solution {
public:vector<vector<int>> result;vector<int> path;void backtracking(int n, int k, int startIndex){if(path.size() == k){result.push_back(path);return;}for(int i = startIndex; i <= n; i ++ ){path.push_back(i);backtracking(n, k, i + 1);path.pop_back();}}vector<vector<int>> combine(int n, int k) {backtracking(n, k, 1);return result;}
};

216. 组合总和 III link

class Solution {
public:vector<vector<int>> result;vector<int> path;void backtracking(int targetSum, int k, int sum, int startIndex){if(path.size() == k){if(sum == targetSum)result.push_back(path);return;}for(int i = startIndex; i <= 9; i ++){sum += i;path.push_back(i);backtracking(targetSum, k, sum, i + 1);sum -= i;path.pop_back();}}vector<vector<int>> combinationSum3(int k, int n) {backtracking(n, k, 0, 1);return result;}
};

17. 电话号码的字母组合link

class Solution {
public:const string letterMap[10] = {"", // 0"", // 1"abc", // 2"def", // 3"ghi", // 4"jkl", // 5"mno", // 6"pqrs", // 7"tuv", // 8"wxyz", // 9};vector<string> result;string s;void backtracking(const string& digits, int index){if(index == digits.size()){result.push_back(s);return;}int digit = digits[index] - '0';string letters = letterMap[digit];for(int i = 0; i < letters.size(); i ++){s.push_back(letters[i]);backtracking(digits, index + 1);s.pop_back();}}vector<string> letterCombinations(string digits) {if(digits.size() == 0){return result;}backtracking(digits, 0);return result;}
};