Leetcode-561. Array Partition I (C Language)

The problem：

Given an array of 2n integers, your task is to group these integers into n pairs of integer, say (a1, b1), (a2, b2), ..., (an, bn) which makes sum of min(ai, bi) for all i from 1 to n as large as possible.

Example 1:

Input: [1,4,3,2]

Output: 4
Explanation: n is 2, and the maximum sum of pairs is 4 = min(1, 2) + min(3, 4).

Note:

1. n is a positive integer, which is in the range of [1, 10000].
2. All the integers in the array will be in the range of [-10000, 10000].

My method: I use the quickSort() when the nums are huge, and insertSort() when the numsSize are small. And make some optimizations

void swap(int *nums, int i, int j) {
        int tmp;

        tmp = nums[i];
        nums[i] = nums[j];
        nums[j] = tmp;
}

int find_prov(int *nums, int low, int high) {
    int m = low + (high - low)/2; //Try to find a middle value from {nums[low], nums[m], nums[high]}
    int key;
    
    if (nums[low] > nums[high]) {
        swap(nums, low, high);
    }
    if (nums[high] < nums[m]) {
        swap(nums, high, m);
    }
    if (nums[m] > nums[low]) {
        swap(nums, m, low);
    }
    key = nums[low]; //keep nums[low] is the middle value.
    
    while (low < high) {
        while (low < high && nums[high] >= key) {
            high--;
        }
        if (low < high) {
            nums[low++] = nums[high];
        }
        while (low < high && nums[low] <= key) {
            low++;
        }
        if (low < high) {
            nums[high--] = nums[low];
        }
    }
    nums[low] = key;
    return low;
}

void quickSort(int* nums, int low, int high) {
    int prov;
    
    if (low < high) {
        prov = find_prov(nums, low, high);
        quickSort(nums, low, prov - 1);
        quickSort(nums, prov + 1, high);
    }
}

void insertSort(int* nums, int len) {
    int i, j;
    int key;
    
    for (i = 1; i < len; i++) {
        key = nums[i];
        for (j = i - 1; j >= 0 && (nums[j] >= key); j--) {
            nums[j + 1] = nums[j];
        }
        nums[j + 1] = key;
    }
}

int arrayPairSum(int* nums, int numsSize) {
    int i, j, k; 
    int sum = 0;

    if (numsSize < 1 || numsSize > 20000) {
        printf("error, pls ensure numsSize in the range of [1, 10000].\n");
        return 0;
    }
    
    int arr1[20000];
    int arr2[20000];
    for (i = 0, j = 0, k = 0; i < numsSize; i++) {//Split two parts: negative and non-negative 
        if (nums[i] < 0) {
            arr1[j++] = nums[i];
        } else {
            arr2[k++] = nums[i];
        }
    }

    if (numsSize > 9) { //For large arr, use quickSort; instead use insertSort
        quickSort(arr1, 0, j - 1);
        quickSort(arr2, 0, k - 1);
    } else {
        insertSort(arr1, j);
        insertSort(arr2, k);
    }
 
    for (i = 0; i < j; i+=2) {
        sum += arr1[i];
    }
    for (i = k - 2; i >= 0; i-=2) {
        sum += arr2[i];
    }
    return sum;

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}

Test results:

Array Partition I

Submission Detail

81 / 81 test cases passed.	Status:  Accepted
Runtime: 28 ms