21.回溯算法3

复原ip地址

class Solution {
public:
    string ip;
    vector<string> res;
    int level=0;
    void backtracking(string s,int start){
        if((level==4)&&(start==s.size())){
            res.push_back(ip.erase(ip.size()-1,1));
            return;
        }else if((level<4)&&(start>s.size())){
            return;
        }
        int sum=0;
        for(int i=start;i<s.size();i++){
        
            if(i==start+1&&s[start]=='0')return;;
            sum=sum*10+(s[i]-'0');
            ip+=s[i];
            if(sum>=0&&sum<=255){
                string tmp=ip;
                ip+=".";
                level++;
                backtracking(s,i+1);
                level--;
                ip=tmp;
            }else{return;}
        }
    }
    vector<string> restoreIpAddresses(string s) {
        backtracking(s,0);
        return res;
    }
};

The error you’re encountering is due to an incorrect operation in the line:

ip += s[i] + ".";

Here, s[i] is a character, and when you add it to the string ".", it doesn’t behave as expected. Instead of appending the character followed by a dot (.), the expression s[i] + "." tries to perform pointer arithmetic because s[i] is treated as an integer (its ASCII value) and "." is treated as a const char*. This leads to undefined behavior, which is causing the runtime error.

所以要分成两步写,当然正确的逻辑是不会写出这句的，因为+s[i]是在循环外面，我是写错的时候发现了这个错误，所以记录一下。

子集

class Solution {
public:
    vector<vector<int>> res;
    void recursive(vector<int>& nums,int start){
        if(start==nums.size()){
            vector<int> tmp;
            
            res.push_back(tmp);
            return;
        }
        recursive(nums,start+1);
        int n=res.size();
        for(int i=0;i<n;i++){
            vector<int> tmp;
            tmp=res[i];
            tmp.push_back(nums[start]);
            res.push_back(tmp);
        }
    }
    vector<vector<int>> subsets(vector<int>& nums) {
        recursive(nums,0);
        return res;
    }
};

子集2

class Solution {
public:
    vector<int> path;
    vector<vector<int>> res;
    void backtracking(vector<int>& nums,int start){
        for(int i=start;i<nums.size();i++){
            if(i>start&&nums[i-1]==nums[i])continue;
            path.push_back(nums[i]);
            backtracking(nums,i+1);
            path.pop_back();
        }
        res.push_back(path);
    }
    vector<vector<int>> subsetsWithDup(vector<int>& nums) {
        sort(nums.begin(),nums.end());
        backtracking(nums,0);
        return res;
    }
};