DAY28|● 93.复原IP地址 ● 78.子集 ● 90.子集II

93.复原IP地址 

class Solution {
private:
    vector<string> result;
    void backtracking(string s,int startindex, int point){
        if(point == 3){
            if (isValid(s, startindex, s.size() - 1)){
                result.push_back(s);
            }
            return;
        }
        for(int i = startindex;i < s.size(); i++){
            if(isValid(s, startindex, i)){
                s.insert(s.begin() + i + 1,'.');
                point++;
                backtracking(s, i+2, point);
                s.erase(s.begin() + i + 1); 
                point--;        
            }else break;
        }
    } 
    bool isValid(string s, int start, int end){
        if(start > end){
            return false;
        }
        if(s[start] == '0' && start != end){
            return false;
        }
        int num = 0;
        for(int i = start;i <= end;i++){
            if(s[i] > '9' || s[i] < 0){
                return false;
            }
            num = num * 10 + (s[i] - '0');
            if(num > 255){
                return false;
            }
        }
        return true;
    }
public:
    vector<string> restoreIpAddresses(string s) {
        if (s.size() < 4 || s.size() > 12) return result; 
        backtracking(s, 0, 0);
        return result;
    }
};

78.子集

求取子集问题，不需要任何剪枝！因为子集就是要遍历整棵树。排列要考虑元素顺序，但是子集不用考虑，[1,2]和[2,1]是一样的。

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

    }

public:
    vector<vector<int>> subsets(vector<int>& nums) {
        vec.clear();
        result.clear();
        backtracking(nums, 0);
        return result;

    }
};

90.子集II

class Solution {
private:
    vector<int> vec;
    vector<vector<int>> result;
    void backtracking(vector<int>& nums, int startIndex, vector<bool>& used){
        result.push_back(vec);
        if(startIndex > nums.size()){
            return;
        }
        for(int i = startIndex; i < nums.size(); i++){
             if (i > 0 && nums[i] == nums[i - 1] && used[i - 1] == false) {
                continue;
            }
            vec.push_back(nums[i]);
            used[i] = true;
            backtracking(nums, i + 1, used);
            used[i] = false;
            vec.pop_back();
        }
    }
public:
    vector<vector<int>> subsetsWithDup(vector<int>& nums) {
        result.clear();
        vec.clear();
        vector<bool> used(nums.size(), false);
        sort(nums.begin(), nums.end()); // 去重需要排序
        backtracking(nums, 0, used);
        return result;

    }
};