先排后找可以让结果更简单

int cmp(const void* a, const void* b){
    return *(int*)a - *(int*)b;
}
int missingNumber(int* nums, int numsSize){
    qsort(nums, numsSize, 4, cmp);
    for(int i=0; i<numsSize; ++i){
        if(nums[i] == i){
            continue;
        }else{
            return i;
        }
    }
    return numsSize;
}