Leetcode 记录（201~300）

实习面试前再完成100题，争取能匀速解释清楚题

204. Count Primes

素数筛

 class Solution {
 public:
     int countPrimes(int n) {
         if(n<=) return ;
         n=n-;
         vector<int> prime(n+,);
         for(int i=;i<=n;i++)
         {
             if(!prime[i])prime[++prime[]]=i;
             for(int j=;j<=prime[]&&prime[j]<=n/i;j++)
             {
                 prime[prime[j]*i]=;
                 if(i%prime[j]==) break;
             }
         }
         return prime[];
     }
 };

 207 210 无向图判断环

topo排序

 class Solution {
 public:
     vector<int> findOrder(int numCourses, vector<pair<int, int>>& prerequisites) {
         int n=prerequisites.size();
         vector<vector<int> > g(numCourses+);
         vector<int> in(numCourses+,);
         vector<int> ans;
         vector<int> ans1;
         pair<int,int> w;
         for(int i=;i<n;i++){
             w=prerequisites[i];
             g[w.second].push_back(w.first);
             in[w.first]++;
         }
         queue<int> q;
         for(int i=;i<numCourses;i++){
             if(in[i]==)    q.push(i);
         }
         int now;
         while(!q.empty()){
             now=q.front();
             q.pop();
             ans.push_back(now);
             for(int i=;i<g[now].size();i++){
                 in[g[now][i]]--;
                 if(in[g[now][i]]==)    q.push(g[now][i]);
             }
         }
         for(int i=;i<numCourses;i++){
             if(in[i]!=)    return NULL;
         }
         return ans;
     }
 };

 208 Tire树，直接抄了一份模板

 class TrieNode
 {
 public:
     TrieNode *next[];
     bool is_word;

     // Initialize your data structure here.
     TrieNode(bool b = false)
     {
         memset(next, , sizeof(next));
         is_word = b;
     }
 };

 class Trie
 {
     TrieNode *root;
 public:
     Trie()
     {
         root = new TrieNode();
     }

     // Inserts a word into the trie.
     void insert(string s)
     {
         TrieNode *p = root;
         for(int i = ; i < s.size(); ++ i)
         {
             if(p -> next[s[i] - 'a'] == NULL)
                 p -> next[s[i] - 'a'] = new TrieNode();
             p = p -> next[s[i] - 'a'];
         }
         p -> is_word = true;
     }

     // Returns if the word is in the trie.
     bool search(string key)
     {
         TrieNode *p = find(key);
         return p != NULL && p -> is_word;
     }

     // Returns if there is any word in the trie
     // that starts with the given prefix.
     bool startsWith(string prefix)
     {
         return find(prefix) != NULL;
     }

 private:
     TrieNode* find(string key)
     {
         TrieNode *p = root;
         for(int i = ; i < key.size() && p != NULL; ++ i)
             p = p -> next[key[i] - 'a'];
         return p;
     }
 };