代码随想录Day28 | 93.复原IP地址 、78.子集 、90.子集II

93.复原IP地址

只需要三个逗点就可以停止

什么是有效段位：

• 段位以0为开头的数字不合法
• 段位里有非正整数字符不合法
• 段位如果大于255了不合法

class Solution {
private:
    vector<string> result;
    void backtracking(string& s, int startIndex, int pointNum) {
        if (pointNum == 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 , '.');  
                pointNum++;
                backtracking(s, i + 2, pointNum); 
                pointNum--;                       
                s.erase(s.begin() + i + 1);     
            } else break;
        }
    }

    bool isValid(const 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) {
        result.clear();
        if (s.size() < 4 || s.size() > 12) return result; 
        backtracking(s, 0, 0);
        return result;
    }
};

78.子集

史上最简单回溯qaq

注意收割节点的语句要写在终止语句前面

class Solution {
private:
    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();
        }
    }
public:
    vector<vector<int>> subsets(vector<int>& nums) {
        result.clear();
        path.clear();
        backtracking(nums, 0);
        return result;
    }
};

90.子集II 

要点之前都讲过了，树层去重

class Solution {
private:
    vector<vector<int>> result;
    vector<int> path;
    void backtracking(vector<int>& nums, int startIndex) {
        result.push_back(path);
        unordered_set<int> uset;
        for (int i = startIndex; i < nums.size(); i++) {
            if (uset.find(nums[i]) != uset.end()) {
                continue;
            }
            uset.insert(nums[i]);
            path.push_back(nums[i]);
            backtracking(nums, i + 1);
            path.pop_back();
        }
    }

public:
    vector<vector<int>> subsetsWithDup(vector<int>& nums) {
        result.clear();
        path.clear();
        sort(nums.begin(), nums.end()); 
        backtracking(nums, 0);
        return result;
    }
};