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• 因为可以重复，传入start_index的值不需要+1了

class Solution {
public:vector<int> path;vector<vector<int>> result;vector<vector<int>> combinationSum(vector<int>& candidates, int target) {backtracking(candidates, target, 0);return result;}void backtracking(vector<int>& candidates, int target, int start_idnex) {int sum = 0;for (auto n : path) {sum += n;}if (sum > target) {return;}if (sum == target) {result.push_back(path);return;}for (int i = start_idnex; i < candidates.size(); i++) {path.push_back(candidates[i]);backtracking(candidates, target, i);path.pop_back();}}
};

• 需要用used数组来确认相同的数字有没有被用过
• used[i - 1] == 0代表这一层中这个数没被取过

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

• statr_index是回文串开始位置，i是回文串结束位置

class Solution {
public:vector<string> path;vector<vector<string>> result;vector<vector<string>> partition(string s) {backtracking(s, 0);return result;}void backtracking(string s, int start_index) {if (start_index >= s.size()) {result.push_back(path);return;}for (int i = start_index; i < s.size(); i++) {if (is_back_word(s, start_index, i)) {string str = s.substr(start_index, i - start_index + 1);path.push_back(str);backtracking(s, i + 1);path.pop_back();}}}bool is_back_word(string s, int start_index, int end_index) {while (start_index < end_index) {if (s[start_index] != s[end_index]) {return false;}start_index++;end_index--;}return true;}
};