Leetcode题解（25）

77. Combinations

题目

分析：求给定数字n,k的组合数，方法是采用深度搜索算法，代码如下（copy网上代码）

 class Solution {
 public:
     void dfs77(vector<vector<int > > &ans, vector<int> subans, int start, int n, int k)
     {
         if (subans.size() == k)
         {
             ans.push_back(subans); return ;
         }
         for (int i = start; i <= n; i++)
         {
             subans.push_back(i);
             dfs77(ans, subans, i + , n, k);
             subans.pop_back(); // 满足一个条件或者该分支递归完后要删除最后一个
         }
     }
     vector<vector<int> > combine(int n, int k) {
         vector<vector<int > > ans;
         if (n < k || k == ) return ans;
         vector<int> subans;
         dfs77(ans, subans, , n, k);
         return ans;
     }
 };

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78. Subsets

题目

分析：求一个集合的所有子集，代码如下（copy网上代码）

 class Solution {
 public:
     vector<vector<int>>res;
     vector<int>ans;
     vector<vector<int>> subsets(vector<int>& nums) {
         if(nums.empty()) return res;
         sort(nums.begin(), nums.end());
         dfs(, ans, nums);
         return res;

     }
     void dfs(int k, vector<int>ans, vector<int> nums){
         res.push_back(ans);
         for(int i = k; i < nums.size(); i++){
             ans.push_back(nums[i]);
             dfs(i + , ans, nums);
             ans.pop_back();
         }
     }
 };

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79. Word Search

题目

分析：看到这类题目，首先就应该想到“回溯法”，代码如下，具体思路见注释：

 class Solution {
     public:
     int m,n;
 public:
     bool exist(vector<vector<char>>& board, string word) {
         if(""==word)
             return true;
         m=board.size();
         n=board[].size();
         vector<vector<bool>> flag(m,vector<bool>(n,false));
         for(int i=;i<m;i++)
         {
             for(int j=;j<n;j++)
             {
                 if(board[i][j]==word[])
                 {
                     flag[i][j]=true;
                     if(HuiSu(board,i,j,word,,flag))
                         return true;
                     else
                         flag[i][j]=false;

                 }
             }
         }
         return false;
     }

     bool HuiSu(const vector<vector<char>>& board,int i,int j,const string& word,int index,vector<vector<bool>>& flag)
     {
         if(word[index+] == '\0')
             return true;

         int ii,jj;
         ii=i;
         jj=j+;
         bool temp;
         if(isTrue(ii,jj))//向右
         {
             if(flag[ii][jj] == false && board[ii][jj] == word[index+])
             {
                 flag[ii][jj] = true;
                 temp = HuiSu(board,ii,jj,word,index+,flag);
                 if(temp)
                     return true;
                 flag[ii][jj] = false;//这一步很重要
             }
         }

         ii=i;
         jj=j-;
         if(isTrue(ii,jj))//向左
         {
             if(flag[ii][jj] == false && board[ii][jj] == word[index+])
             {
                 flag[ii][jj] = true;
                 temp = HuiSu(board,ii,jj,word,index+,flag);
                 if(temp)
                     return true;
                 flag[ii][jj] = false;
             }
         }

         ii=i-;
         jj=j;
         if(isTrue(ii,jj))//向上
         {
             if(flag[ii][jj] == false && board[ii][jj] == word[index+])
             {
                 flag[ii][jj] = true;
                 temp = HuiSu(board,ii,jj,word,index+,flag);
                 if(temp)
                     return true;
                 flag[ii][jj] = false;
             }
         }

         ii=i+;
         jj=j;
         if(isTrue(ii,jj))//向下
         {
             if(flag[ii][jj] == false && board[ii][jj] == word[index+])
             {
                 flag[ii][jj] = true;
                 temp = HuiSu(board,ii,jj,word,index+,flag);
                 if(temp)
                     return true;
                 flag[ii][jj] = false;
             }
         }

         return false;
     }

     bool isTrue(int i,int j)
     {
         if(i<)
             return false;
         else if(i>=m)
             return false;
         else if(j<)
             return false;
         else if(j>=n)
             return false;

         return true;
     }
 };