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）

 1 class Solution {
 2 public:
 3     void dfs77(vector<vector<int > > &ans, vector<int> subans, int start, int n, int k)
 4     {
 5         if (subans.size() == k)
 6         {
 7             ans.push_back(subans); return ;
 8         }
 9         for (int i = start; i <= n; i++)
10         {
11             subans.push_back(i);
12             dfs77(ans, subans, i + 1, n, k);
13             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
14         }
15     }
16     vector<vector<int> > combine(int n, int k) {
17         vector<vector<int > > ans;
18         if (n < k || k == 0) return ans;
19         vector<int> subans;
20         dfs77(ans, subans, 1, n, k);
21         return ans;
22     }
23 };

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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）

 1 class Solution {
 2 public:
 3     vector<vector<int>>res;
 4     vector<int>ans;
 5     vector<vector<int>> subsets(vector<int>& nums) {
 6         if(nums.empty()) return res;
 7         sort(nums.begin(), nums.end());
 8         dfs(0, ans, nums);
 9         return res;
10      
11     }
12     void dfs(int k, vector<int>ans, vector<int> nums){
13         res.push_back(ans);
14         for(int i = k; i < nums.size(); i++){
15             ans.push_back(nums[i]);
16             dfs(i + 1, ans, nums);
17             ans.pop_back();
18         }
19     }
20 };

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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：

  1 class Solution {
  2     public:
  3     int m,n;
  4 public:
  5     bool exist(vector<vector<char>>& board, string word) {
  6         if(""==word)
  7             return true;
  8         m=board.size();
  9         n=board[0].size();
 10         vector<vector<bool>> flag(m,vector<bool>(n,false));
 11         for(int i=0;i<m;i++)
 12         {
 13             for(int j=0;j<n;j++)
 14             {
 15                 if(board[i][j]==word[0])
 16                 {
 17                     flag[i][j]=true;
 18                     if(HuiSu(board,i,j,word,0,flag))
 19                         return true;
 20                     else
 21                         flag[i][j]=false;
 22                     
 23                     
 24                     
 25                 }
 26             }
 27         }
 28         return false;
 29     }
 30     
 31     bool HuiSu(const vector<vector<char>>& board,int i,int j,const string& word,int index,vector<vector<bool>>& flag)
 32     {
 33         if(word[index+1] == '')
 34             return true;
 35         
 36         int ii,jj;
 37         ii=i;
 38         jj=j+1;
 39         bool temp;
 40         if(isTrue(ii,jj))//向右
 41         {
 42             if(flag[ii][jj] == false && board[ii][jj] == word[index+1])
 43             {
 44                 flag[ii][jj] = true;
 45                 temp = HuiSu(board,ii,jj,word,index+1,flag);
 46                 if(temp)
 47                     return true;
 48                 flag[ii][jj] = false;//这一步很重要
 49             }
 50         }
 51         
 52         ii=i;
 53         jj=j-1;
 54         if(isTrue(ii,jj))//向左
 55         {
 56             if(flag[ii][jj] == false && board[ii][jj] == word[index+1])
 57             {
 58                 flag[ii][jj] = true;
 59                 temp = HuiSu(board,ii,jj,word,index+1,flag);
 60                 if(temp)
 61                     return true;
 62                 flag[ii][jj] = false;
 63             }
 64         }
 65         
 66         ii=i-1;
 67         jj=j;
 68         if(isTrue(ii,jj))//向上
 69         {
 70             if(flag[ii][jj] == false && board[ii][jj] == word[index+1])
 71             {
 72                 flag[ii][jj] = true;
 73                 temp = HuiSu(board,ii,jj,word,index+1,flag);
 74                 if(temp)
 75                     return true;
 76                 flag[ii][jj] = false;
 77             }
 78         }
 79         
 80         ii=i+1;
 81         jj=j;
 82         if(isTrue(ii,jj))//向下
 83         {
 84             if(flag[ii][jj] == false && board[ii][jj] == word[index+1])
 85             {
 86                 flag[ii][jj] = true;
 87                 temp = HuiSu(board,ii,jj,word,index+1,flag);
 88                 if(temp)
 89                     return true;
 90                 flag[ii][jj] = false;
 91             }
 92         }
 93         
 94         
 95         return false;
 96     }
 97     
 98     bool isTrue(int i,int j)
 99     {
100         if(i<0)
101             return false;
102         else if(i>=m)
103             return false;
104         else if(j<0)
105             return false;
106         else if(j>=n)
107             return false;
108         
109         return true;
110     }
111 };