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<=2) return 0;
        n=n-1;
        vector<int> prime(n+1,0);
        for(int i=2;i<=n;i++)
        {
            if(!prime[i])prime[++prime[0]]=i;
            for(int j=1;j<=prime[0]&&prime[j]<=n/i;j++)
            {
                prime[prime[j]*i]=1;
                if(i%prime[j]==0) break;
            }
        }
        return prime[0];
    }
};

 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+1);
        vector<int> in(numCourses+2,0);
        vector<int> ans;
        vector<int> ans1;
        pair<int,int> w;
        for(int i=0;i<n;i++){
            w=prerequisites[i];
            g[w.second].push_back(w.first);
            in[w.first]++;
        }
        queue<int> q;
        for(int i=0;i<numCourses;i++){
            if(in[i]==0)    q.push(i);
        }
        int now;
        while(!q.empty()){
            now=q.front();
            q.pop();
            ans.push_back(now);
            for(int i=0;i<g[now].size();i++){
                in[g[now][i]]--;
                if(in[g[now][i]]==0)    q.push(g[now][i]);
            }
        }
        for(int i=0;i<numCourses;i++){
            if(in[i]!=0)    return NULL;
        }
        return ans;
    }
};

 208 Tire树，直接抄了一份模板

class TrieNode
{
public:
    TrieNode *next[26];
    bool is_word;
    
    // Initialize your data structure here.
    TrieNode(bool b = false)
    {
        memset(next, 0, 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 = 0; 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 = 0; i < key.size() && p != NULL; ++ i)
            p = p -> next[key[i] - 'a'];
        return p;
    }
};