C语言刷2数/3数/4数之和

15. 三数之和

/**
 * Return an array of arrays of size *returnSize.
 * The sizes of the arrays are returned as *returnColumnSizes array.
 * Note: Both returned array and *columnSizes array must be malloced, assume caller calls free().
 */

int Cmp(const void *a,const void *b){
    return *(int*)a - *(int*)b;
}

int** threeSum(int* nums, int numsSize, int* returnSize, int** returnColumnSizes)
{
    *returnSize = 0;
    if (numsSize < 3) {
        return NULL;
    }

    qsort(nums, numsSize, sizeof(int), Cmp);

    int k, i, j, sum;
    int **res = (int **)malloc(sizeof(int *) * 20000);
    *returnColumnSizes = (int *)malloc(sizeof(int) * 20000);
    for (k = 0; k < numsSize - 2; k++) {
        if (nums[k] > 0) {
            return res;
        }
        if (k > 0 && nums[k] == nums[k - 1]) {
            continue;
        }
        i = k + 1;
        j = numsSize - 1;
        while (i < j) {
            sum = nums[k] + nums[i] + nums[j];
            if (sum == 0) {
                // printf("k:%d, i:%d, j:%d, *returnSize:%d\n", k, i, j, *returnSize);
                res[*returnSize] = (int *)malloc(sizeof(int) * 3);
                (*returnColumnSizes)[*returnSize] = 3;
                res[*returnSize][0] = nums[k];
                res[*returnSize][1] = nums[i];
                res[*returnSize][2] = nums[j];
                *returnSize += 1;
                // printf("1 i:%d, j:%d\n", i, j);
                // 更新和去重
                while(i < j && nums[i] == nums[++i]) {
                }
                // printf("2 i:%d, j:%d\n", i, j);
                // 更新和去重
                while(i < j && nums[j] == nums[--j]) {
                }
                // printf("3 i:%d, j:%d\n", i, j);
            } else if (sum < 0) {
                i++;
            } else {
                j--;
            }
        }
    } 

    return res;
}   

16. 最接近的三数之和

int Cmp(const void *a, const void *b)
{
    return *(int *)a - *(int *)b;
}

int threeSumClosest(int* nums, int numsSize, int target)
{
    // 排序
    qsort(nums, numsSize, sizeof(int), Cmp);

    int k, i, j, sum, best, sub;
    best = 1e7; // 用一个很大的数，保证一定会更新1次best
    for (k = 0; k < numsSize - 2; k++) {
        if (k > 0 && nums[k] == nums[k - 1]) {
            continue;
        }
        i = k + 1;
        j = numsSize - 1;
        while (i < j) {
            sum = nums[k] + nums[i] + nums[j];
            if (sum == target) {
                return sum;
            }
            if (abs(sum - target) < abs(best - target)) {
                best = sum;
            }

            if (sum < target) {
                i++;
                if (i == j) {
                    break;
                }
            } else if (sum > target) {
                j--;
                if (i == j) {
                    break;
                }
            }
        }
    }
    return best;
}