最大子段和

//最大字段和{-1，3，-1，6，-3}//1 枚举法//列举所有的字段//以-1开始的{-1}，{-1，3}，{-1，3，-1}。。。//以-3开始的{-3}。int max_sum(int *arr, int n){    int sum = arr[0];    for (int i = 0; i < n; ++i){        int this_sum = 0;        for (int j = i; j < n; ++j){            this_sum += arr[j];            if (this_sum > sum){                sum = this_sum;            }        }    }    return sum;}int max_sum_range(int* arr, int n, int& first, int& last){    int sum = arr[0];    first = last = 0;    for (int i = 0; i < n; ++i){        int this_sum = 0;        for (int j = i; j < n; ++j){            this_sum += arr[j];            if (this_sum > sum){                sum = this_sum;                first = i;                last = j;   //记录区间            }        }    }    return sum;}//动态规划//用this_sum[]存储当前的字段最大和//max存储历史最大//对每次更新this_sum[i]来说，如果this_sum[i-1] < 0则this_sum[i]=this_sum[i-1]//且当前的first = i//如果this_sum[i-1] >= 0 则this_sum[i] = this_sum[i-1] + a[i];//更新max来说 当this_sum[i] > max//max = this_sum[i]//first= 当前first//last = i;int max_sum_range_2(int* arr, int n, int& first, int& last){    int* this_sum = new int[n];    int  max = arr[0];    first = last = 0;    int min;    for(int i = 1; i < n;++i){        if(this_sum[i-1] >= 0){            this_sum[i] = this_sum[i-1] + arr[i];        }else{            min = i;            this_sum[i] = arr[i];        }        if(this_sum[i] > max){            max = this_sum[i];            last = i;            first = min;        }    }    delete[] this_sum;    return max;}int main(){    int arr[4]{-9, 14, -17, -18};    cout << "枚举" << endl;    cout << max_sum(arr, 4) << endl;    int first, last;    cout <<endl<< "枚举" << endl;    cout << max_sum_range(arr, 4, first, last) << endl;    for (int i = first; i <= last; ++i){        cout << arr[i] << ends;    }    cout << endl<< "动规" << endl;    cout << max_sum_range_2(arr, 4, first, last) << endl;    for (int i = first; i <= last; ++i){        cout << arr[i] << ends;    }    return 0;}