代码片段收集

一些自己用得比较顺手的代码

定义 Compare

排序有时需要自己指定比较器 (Comparator)

# 指定std::greater<> 作为比较器，将vector或者别的容器降续排列

#include<functional> //greateer
#include<iostream>
#include<vector>
#include<queue>
#include<algorithm>

int main(){
    int A[] = {6, 9, 1, 5, 2};
    std::vector<int> vec(A, A+sizeof(A)/sizeof(A[0])); 
    std::sort(vec.begin(), vec.end(), std::greater<int>()); //将vector降续排列，注意这里greater<int> 后有要加括号()
    for(int i = 0; i < vec.size(); std::cout << vec[i++] << ' '); //9 6 5 2 1
    std::cout << std::endl;

    std::priority_queue<int, std::vector<int>, std::greater<int> > pqueue(A, A+sizeof(A)/sizeof(A[0])); //定义最小堆，注意这里greater<int> 后没有括号
    while(!pqueue.empty()){
        std::cout << pqueue.top() << ' '; //1 2 5 6 9
        pqueue.pop();
    }
    std::cout << std::endl;

    return 0;
}

# 自定义比较器，让优先队列 可以存储特殊struct。

#include<iostream>
#include<vector>
#include<queue>
#include<utility> //pair
using namespace std;

typedef pair<int, int> Node;

class Comparator{
public:
    bool operator()(const Node &a, const Node &b){
        return a.second < b.second;    //升序，队头元素最小
    }
};

int main(){
    int A[] = {9, 2, 8, 7, 0, 6};
    priority_queue<Node, vector<Node>, Comparator> pqueue;
    for(int i = 0; i < sizeof(A) / sizeof(int); pqueue.push(Node(i, A[i])), ++i);
    while(!pqueue.empty()){
        cout << pqueue.top().second << ", orginal index: " << pqueue.top().first << endl;
        pqueue.pop();
    }

    return 0;
}


/*
输出：
9, orginal index: 0
8, orginal index: 2
7, orginal index: 3
6, orginal index: 5
2, orginal index: 1
0, orginal index: 4
*/

max_element() 与 min_element()

max_element() 可以得出序列中最大值所在的iterator

#include <iostream>     // std::cout
#include <algorithm>    // std::min_element, std::max_element

bool myfn(int i, int j) { return i<j; }

struct myclass {
  bool operator() (int i,int j) { return i<j; }
} myobj;

int main () {
  int myints[] = {3,7,2,5,6,4,9};

  // using default comparison:
  std::cout << "The smallest element is " << *std::min_element(myints,myints+7) << '
';
  std::cout << "The largest element is "  << *std::max_element(myints,myints+7) << '
';

  // using function myfn as comp:
  std::cout << "The smallest element is " << *std::min_element(myints,myints+7,myfn) << '
';
  std::cout << "The largest element is "  << *std::max_element(myints,myints+7,myfn) << '
';

  // using object myobj as comp:
  std::cout << "The smallest element is " << *std::min_element(myints,myints+7,myobj) << '
';
  std::cout << "The largest element is "  << *std::max_element(myints,myints+7,myobj) << '
';

  return 0;
}

链表类

# 需要一个指针走一步，一个指针走两步。

for(; p2 != NULL; p1 = p1 -> next, p2 = p2 -> next){
            p2 = p2 -> next;
            if(p2 == NULL) break;
        }

格式的特点是 p1 = p1 -> next, p2 = p2 -> next 写在 for 的里面，为了防止空指针引用，for循环体里要加上 if(p2 == NULL) break;

场景(1) 将链表分为两段，前半段长度 n - n/2， 后半段长度n/2

        ListNode *p1 = head; ListNode* p2 = head -> next;
        for(; p2 != NULL;p1 = p1 -> next, p2 = p2 -> next){
            p2 = p2 -> next;
            if(p2 == NULL) break;
        }
        ListNode* temp = p1;
        p1 = p1 -> next;
        temp -> next = NULL;

场景(2) 除了分两段，判断环链表也需要这种格式的代码，之所以要从head -> next出发，是因为如果要接着找出环入口的话，相遇点的位置是正确的。

bool hasCycle(ListNode *head) {
        if(NULL == head) return false;
        
        ListNode* p1 = head -> next; ListNode* p2 = head -> next;
        for(; p2 != NULL; p1 = p1 -> next, p2 = p2 -> next){
            p2 = p2 -> next;
            if(p2 == NULL || p1 == p2) break;
        }
        if(p2 == NULL) return false;
        return true;
    }

话说回来，这样写稍微有点非主流，清晰一点的写法是这样：

ListNode *FindCircleStart(ListNode *head){
    if(!head) return NULL;

    ListNode *p = head, *q = head;
    while(q -> next){
        p = p -> next;
        q = q -> next -> next;
        if(!q || q == p) break;
    }
    if(!q) return NULL; //无环
    for(p = head; p!= q; p = p -> next, q = q -> next); //找出相遇点
    return p;
}

看似简单，但是其实考虑到了几种边界情况，比如只有一个节点的环链表。

BIT 类

返回一个整数转化为二进制后，1 的个数

int __builtin_popcount(unsigned int)
int __builtin_popcountl(unsigned long int)
int __builtin_popcountll(unsigned long long)

这三个函数是GCC提供的函数，不属于std

要想自己实现的话，有一个效率比较高的方法

http://www.cnblogs.com/felixfang/category/535191.html 后半部分

数组类 

排好序的数组中去重

Given input array A = [1,1,2],

Your function should return length = 2, and A is now [1,2].

int removeDuplicates(int A[], int n) {
        if(n <= 1) return n;
        int p = 0, q = 1;
        while(q < n){
            if(A[p] == A[q]) ++q;
            else A[++p] = A[q++];
        }
        return p+1;
    }

如果重复数字允许出现两次

Given sorted array A = [1,1,1,2,2,3],

Your function should return length = 5, and A is now [1,1,2,2,3].

只要在++q的条件上增加两项即可。

int removeDuplicates(int A[], int n) {
        if(n <= 1) return n;
        int p = 0, q = 1;
        while(q < n){
            if(A[p] == A[q] && p > 0 && A[p] == A[p-1]) ++q;
            else A[++p] = A[q++];
        }
        return p+1;
    }

 

 字符串处理类

string a;
char* chs = a.c_str(); //string to char*
int num = atoi(a.c_str()) //string to int

#include <string>
#include <sstream>

string convert(int v){
    ostringstream convert;   // stream used for the conversion
    convert << v;      // insert the textual representation of 'Number' in the characters in the stream
    return convert.str(); // set 'Result' to the contents of the stream
}