#include <iostream>

using namespace std;

// 迭代求和
int mySum(int a[], int n){
    int sum = 0;
    for(int i=0; i<n; i++){
        sum += a[i];
    }
    return sum;
}

// 递归求和
int toSum(int a[], int n){
    return n < 1 ? 0 : toSum(a,n-1) + a[n-1];
}

// 迭代反转
void myReverse(int a[],int lo,int hi){
    while(lo<hi){
        swap(a[lo++],a[hi--]);
    }
}

// 递归反转
void toReverse(int a[],int lo,int hi){
    if(lo<hi){
        swap(a[lo],a[hi]);
        toReverse(a,lo+1, hi-1);
    }
}

int main()
{
    int A[15]={1,2,3,4,5,6,7,8,9};
    cout << "原数列为：" << endl;
    for(int j=0;j<9;j++)
        cout << A[j] <<" ";

    cout << "" << endl;
    int sum1 = mySum(A,9);
    cout << "迭代求和结果：" << sum1 << endl;
    int sum2 = toSum(A,9);
    cout << "递归求和结果：" << sum2 << endl;

    myReverse(A,0,8);
    cout << "迭代反转数列A为：" << endl;
    for(int j=0;j<9;j++)
        cout << A[j] <<" ";

    cout << "" << endl;

    toReverse(A,0,8);
    cout << "递归再次反转回数列为：" << endl;
    for(int j=0;j<9;j++)
        cout << A[j] <<" ";

    cout << "" << endl;

    cout << "操作结束!" << endl;
    return 0;
}

#include <iostream>

using namespace std;

// 二分递归求和
int binarySum(int a[], int lo, int hi){
    if(lo + 1 >= hi) return a[lo];
    int mi = (lo + hi) >> 1;
    return binarySum(a,lo,mi) + binarySum(a,mi,hi);
}

int main()
{
    int A[15]={1,2,3,4,5,6,7,8};
    cout << "原数列为：" << endl;
    for(int j=0;j<8;j++)
        cout << A[j] <<" ";
    cout << "" << endl;

    // 注意这里的输入区间为(0,n)，否则计算会出错
    int sum0 = binarySum(A,0,8);
    cout << "数列的和为：" <<sum0<<endl;
    cout << "操作结束!" << endl;
    return 0;
}

#include <iostream>

using namespace std;

// 迭代找出数组最大两数
void myMax2(int a[], int lo, int hi, int & x1, int & x2){
    if(a[x1=lo] < a[x2=lo+1]) swap(x1, x2);
    for(int i=lo+2; i<hi; i++){
        if(a[x2] < a[i]){
            if(a[x1] < a[x2=i])
                swap(x1, x2);
        }
    }
}

// 递归分治找出数组最大两数
void toMax(int a[], int lo, int hi, int & X1, int & X2){
    if(hi <= lo+3){myMax2(a, lo, hi, X1, X2); return ;}
    int mi = (lo+hi) / 2;
    int x1L, x2L; toMax(a, lo, mi, x1L, x2L);
    int x1R, x2R; toMax(a, mi, hi, x1R, x2R);
    if(a[x1L] > a[x1R]){
        X1 = x1L; X2 = a[x2L] > a[x1R] ? x2L : x1R;
    }else{X1 = x1R; X2 = a[x2R] > a[x1L] ? x2R : x1L;}
}

int main()
{
    int x1,x2,c1,c2;

    int A[15]={4,5,3,7,6,9,2,1};
    cout << "原数列为：" << endl;
    for(int j=0;j<8;j++)
        cout << A[j] <<" ";
    cout << "" << endl;

    // 注意这里区间为(0,n)
    myMax2(A,0,8,x1,x2);
    cout << "迭代过程--" << "最大数为：" << A[x1] << "  次大数为：" << A[x2] << endl;

    // 注意这里区间同样为(0,n)
    toMax(A,0,8,c1,c2);
    cout << "递归过程--" << "最大数为：" << A[x1] << "  次大数为：" << A[x2] << endl;

    cout << "操作结束!" << endl;
    return 0;
}