Leetcode 回溯算法（二）

39. 组合总和

力扣题目链接

class Solution {
public:
    vector<vector<int>> result;
    vector<int> path;
    void backtracking(vector<int>& candidates, int target, int sum, int startIndex) {
        if(sum > target)    return;
        if(sum == target){
            result.push_back(path);
            return;
        }

        for(int i = startIndex; i <candidates.size(); i++){
            sum += candidates[i];
            path.push_back(candidates[i]);
            backtracking(candidates, target, sum, i);
            sum -= candidates[i];
            path.pop_back();
        }
    }
    vector<vector<int>> combinationSum(vector<int>& candidates, int target) {
        result.clear();
        path.clear();
        backtracking(candidates, target, 0, 0);
        return result;
    }
};

40. 组合总和 II

力扣题目链接

class Solution {
public:
    vector<vector<int>> result;
    vector<int> path;
    void backtracking(vector<int>& candidates, int target, int sum, int startIndex, vector<bool>& used) {
        if(sum == target){
            result.push_back(path);
            return;
        }
        for(int i = startIndex; i < candidates.size() && sum + candidates[i] <= target; i++){
            if(i > 0 && candidates[i] == candidates[i - 1] && used[i - 1] == false) continue;
            sum += candidates[i];
            path.push_back(candidates[i]);
            used[i] = true;
            backtracking(candidates, target, sum, i + 1, used); 
            used[i] = false;
            sum -= candidates[i];
            path.pop_back();
        }
    }
    vector<vector<int>> combinationSum2(vector<int>& candidates, int target) {
        vector<bool> used(candidates.size(), false);
        path.clear();
        result.clear();
        sort(candidates.begin(),candidates.end());
        backtracking(candidates, target, 0, 0, used);
        return result;
    }
};

131. 分割回文串

力扣题目链接

class Solution {
public:
    vector<vector<string>> result;
    vector<string> path;
    void backtracking(const string& s, int startIndex) {
        if(startIndex >= s.size()){
            result.push_back(path);
            return;
        }
        for(int i = startIndex; i < s.size(); i++){
            if(isPalindrome(s, startIndex, i)){
                string str = s.substr(startIndex, i - startIndex + 1);
                path.push_back(str);
            }
            else    continue;
            backtracking(s, i + 1);
            path.pop_back();
        }
    }
    bool isPalindrome(const string& s, int start, int end) {
        for(int i = start, j = end; i < j ; i++, j--){
            if(s[i] != s[j])    return false;
        }
        return true;
    }
    vector<vector<string>> partition(string s) {
        result.clear();
        path.clear();
        backtracking(s, 0);
        return result;
    }
};

78. 子集

力扣题目链接

class Solution {
public:
    vector<vector<int>> result;
    vector<int> path;
    void backtracking(vector<int>& nums, int startIndex){
        result.push_back(path);
        if(startIndex >= nums.size())   return;
        for(int i = startIndex; i < nums.size(); i++){
            path.push_back(nums[i]);
            backtracking(nums, i + 1);
            path.pop_back();
        }
    }
    vector<vector<int>> subsets(vector<int>& nums) {
        result.clear();
        path.clear();
        backtracking(nums, 0);
        return result;
    }
};