map按key和按value排序

//golang的map不保证有序性，所以按key排序需要取出key，对key排序，再遍历输出value

package main



import (

    "fmt"

    "sort"

)



func main() {

    // To create a map as input

    m := make(map[int]string)

    m[1] = "a"

    m[2] = "c"

    m[0] = "b"



    // To store the keys in slice in sorted order

    var keys []int

    for k := range m {

        keys = append(keys, k)

    }

    sort.Ints(keys)



    // To perform the opertion you want

    for _, k := range keys {

        fmt.Println("Key:", k, "Value:", m[k])

    }

}

//要对golang map按照value进行排序，思路是直接不用map，用struct存放key和value，实现sort接口，就可以调用sort.Sort进行排序了。

// A data structure to hold a key/value pair.

type Pair struct {

    Key   string

    Value int

}



// A slice of Pairs that implements sort.Interface to sort by Value.

type PairList []Pair



func (p PairList) Swap(i, j int)      { p[i], p[j] = p[j], p[i] }

func (p PairList) Len() int           { return len(p) }

func (p PairList) Less(i, j int) bool { return p[i].Value < p[j].Value }



// A function to turn a map into a PairList, then sort and return it.

func sortMapByValue(m map[string]int) PairList {

    p := make(PairList, len(m))

    i := 0

    for k, v := range m {

        p[i] = Pair{k, v}

    }

    sort.Sort(p)

    return p

}

//sort.Sort是递增排序，如果要实现递减排序，用sort.Reverse

package main



import (

    "fmt"

    "sort"

)



func main() {

    a := []int{4,3,2,1,5,9,8,7,6}

    sort.Sort(sort.Reverse(sort.IntSlice(a)))

    fmt.Println("After reversed: ", a)

}

//sort不保证排序的稳定性（两个相同的值，排序之后相对位置不变），排序的稳定性由sort.Stable来保证。

package main



import (

    "fmt"

    "sort"

)



type person struct {

  Name string

  Age int

}



type personSlice []person



func (s personSlice) Len() int { return len(s) }

func (s personSlice) Swap(i, j int) { s[i], s[j] = s[j], s[i] }

func (s personSlice) Less(i, j int) bool { return s[i].Age < s[j].Age }



func main() {

    a := personSlice {

      {

        Name: "AAA",

        Age: 55,

      },

      {

        Name: "BBB",

        Age: 22,

      },

      {

        Name: "CCC",

        Age: 0,

      },

      {

        Name: "DDD",

        Age: 22,

      },

      {

        Name: "EEE",

        Age: 11,

      },  

    }

    sort.Stable(a)

    fmt.Println(a)

}

/看一下本题的C++解法，C++ sort的第三个参数用来定义排序方法，即按key还是value排序，递增还是递减排序等，stable_sort用来保证排序的稳定性，主要思路与golang解法相似，都是用struct封装key和value来代替map。

#include <iostream>

#include <vector>

#include <algorithm>

#include <string>

using namespace std;

struct student{

  string name;

    int score;

};

bool cmp0(const student &a, const student &b){

    // 从高到低排序

    return a.score > b.score;

}

bool cmp1(const student &a, const student &b){

    // 从低到高排序

    return a.score < b.score;

}

int main(){

    //freopen("in.txt", "r", stdin);

    //freopen("out.txt", "w", stdout);

    int N, type;

    while(cin >> N >> type){

        vector<student> stud(N);

 

        for(int i = 0; i < N; i ++){

            cin >> stud[i].name >> stud[i].score;

        }

        if(type == 0)

            stable_sort(stud.begin(), stud.end(), cmp0);   //稳定排序

        else

            stable_sort(stud.begin(), stud.end(), cmp1);

 

        for(int i = 0; i < N; i ++){

            cout << stud[i].name << " " << stud[i].score << endl;

        }

    }

 

    return 0;

}