day20打卡

1. 复原IP地址

   class Solution {
   public:
   bool isValid(string s, int start, int end)
   {
   if (start > end) {
   return false;
   }
   if (s[start] == '0' && start != end) { // 0开头的数字不合法
   return false;
   }
   int num = 0;
   for (int i = start; i <= end; i++) {
   if (s[i] > '9' || s[i] < '0') { // 遇到非数字字符不合法
   return false;
   }
   num = num * 10 + (s[i] - '0');
   if (num > 255) { // 如果大于255了不合法
   return false;
   }
   }
   return true;
   }
   void backtracking(string s, int start)
   {
   if(start == s.size())
   {
   if(path.size() == 4)
   {
   string str;
   str += path[0];
   for(int i = 1; i < 4; ++i)
   {
   str += "." + path[i];
   }
   ret.push_back(str);
   }
   return;
   }
   for(int i = start; i < s.size(); ++i)
   {
   if(isValid(s, start, i))
   {
   path.push_back(s.substr(start, i-start+1));
   }
   else
   {
   continue;
   }
   backtracking(s, i+1);
   path.pop_back();
   }
   }
   vector restoreIpAddresses(string s) {
   backtracking(s, 0);
   return ret;
   }
   private:
   vector path;
   vector ret;
   };

2. 子集

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

3. 子集II

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