【C++ 】sort利用函数排序

https://leetcode-cn.com/problems/ba-shu-zu-pai-cheng-zui-xiao-de-shu-lcof/

补记：应注意：

sort(first,last,cmp):

中的cmp在编写自定义比较的时候，相等情况应当返回false，不然会出bug。

https://en.cppreference.com/w/cpp/named_req/Compare中写道：

严格是说在判断的时候会用"<"，而不是"<="

1、sort的简单应用：

sort - C++ Reference (cplusplus.com)

// sort algorithm example
#include <iostream>     // std::cout
#include <algorithm>    // std::sort
#include <vector>       // std::vector

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

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

int main() {
    int myints[] = { 32,71,12,45,26,80,53,33 };
    std::vector<int> myvector(myints, myints + 8);               // 32 71 12 45 26 80 53 33

    // using default comparison (operator <):
    std::sort(myvector.begin(), myvector.begin() + 4);           //(12 32 45 71)26 80 53 33

    // using function as comp
    std::sort(myvector.begin() + 4, myvector.end(), myfunction); // 12 32 45 71(26 33 53 80)

    // using object as comp
    std::sort(myvector.begin(), myvector.end(), myobject);     //(12 26 32 33 45 53 71 80)

    // print out content:
    std::cout << "myvector contains:";
    for (std::vector<int>::iterator it = myvector.begin(); it != myvector.end(); ++it)
        std::cout << ' ' << *it;
    std::cout << '\n';

    return 0;
}

2、cmp函数的理解

返回值为true，vector等容器按照调用顺序进行排列，否则反序。

这里面比较难理解的是利用myobject和myfunction进行建立排序规则。理解是：myobject、myfunction中的i，j都是待排序的对象，其形参则是

sort(x.begin(), x.end(),....);

中"x"的一个元素。

即对于sort(first,last,cmp):

first, lastRandom-access iterators to the initial and final positions of the sequence to be sorted. The range used is [first,last), which contains all the elements between first and last, including the element pointed by first but not the element pointed by last.

RandomAccessIterator

shall point to a type for which

swap

is properly defined and which is both move-constructible and move-assignable.
compBinary function that accepts two elements in the range as arguments, and returns a value convertible to bool. The value returned indicates whether the element passed as first argument is considered to go before the second in the specific strict weak ordering it defines.
The function shall not modify any of its arguments.
This can either be a function pointer or a function object.

如果对于一个二维vector的每一行二元数组的第二个元素进行排序，代码是这么写的：

435. 无重叠区间 - 力扣（LeetCode） (leetcode-cn.com)

class Solution {
public:
    // 按照区间右边界排序
    static bool cmp (const vector<int>& a, const vector<int>& b) {
        return a[1] < b[1];
    }
    int eraseOverlapIntervals(vector<vector<int>>& intervals) {
        if (intervals.size() == 0) return 0;
        sort(intervals.begin(), intervals.end(), cmp);
        int count = 1; // 记录非交叉区间的个数
        int end = intervals[0][1];
        for (int i = 1; i < intervals.size(); i++) {
            if (end <= intervals[i][0]) {
                end = intervals[i][1];
                count++;
            }
        }
        return intervals.size() - count;
    }
};