怎样合并排序数组（How to merge 2 sorted arrays?）

Question: We have 2 sorted arrays and we want to combine them into a single sorted array.

Input: arr1[] = 1, 4, 6, 8, 13, 25    ||     arr2[] = 2, 7, 10, 11, 19, 50
Output: 1, 2, 4, 6, 7, 8, 10, 11, 13, 19, 50

最简单的方法之一就是把两个数组复制到一个新的数组中，对这个新的数组进行排序。但这样就不能利用原来的两个数组已经排好序这个条件了。

我们需要一个不一样的方法。下面是可行方法之一：

• 为两个数组初始化两个变量作为索引。
• 假设i指向arr1[]，j指向arr2[]。
• 比较arr1[i]，arr2[j]，哪个小就将那个复制进新的数组，并增加相应的系数。
• 重复上述步骤直到i和j都到达数组尾部。

相应的算法实现：

#include<stdio.h>

//a function to merge two arrays
//array1 is of size 'l'
//array2 is of size 'm'
//array3 is of size n=l+m
void merge(int arr1[], int arr2[], int arr3[], int l, int m, int n)
{
    //3 counters to point at indexes of 3 arrays
    int i,j,k;
    i=j=k=0;

    //loop until the array 1 and array 2 are within bounds
    while(i<l && j<m)
    {
        //find the smaller element among the two
        //and increase the counter
        if(arr1[i] < arr2[j])
        {
            arr3[k] = arr1[i];

            //increment counter of 1st array
            i++;
        }
        else
        {
            arr3[k] = arr2[j];

            //increment counter of second array
            j++;
        }

        //increase the counter of the final array
        k++;
    }

    //now fill the remaining elements as it is since they are
    //already sorted
    while(i<l)
    {
        arr3[k] = arr1[i];
        i++;
        k++;
    }
    while(j<m)
    {
        arr3[k] = arr2[j];
        j++;
        k++;
    }
}

//driver program to test the above function
int main(void)
{
    int arr1[5] = {1, 5, 9, 11, 15};
    int arr2[5] = {2, 4, 13, 99, 100};

    int arr3[10] = {0};

    merge(arr1, arr2, arr3, 5, 5, 10);

    int i=0;
    for(i=0;i<10;i++)
        printf("%d ",arr3[i]);

    return 0;
}