LeetCode:Find K Pairs with Smallest Sums

This problem is difficult for me.

class Solution(object):
    def kSmallestPairs(self, nums1, nums2, k):
        """
        :type nums1: List[int]
        :type nums2: List[int]
        :type k: int
        :rtype: List[List[int]]
        """
               
        tables = ([[1 for _ in range(len(nums2) + 1)]] +
                  [[1] + [0 for _ in range(len(nums2))] for _ in range(len(nums1))])
        # print(tables)
        def read_tables(i1,i2):
            return tables[i1+1][i2+1]
        
        def write_tables(i1,i2,num):
            tables[i1+1][i2+1] = num
            
        def push_output_and_write_table(idx1,idx2):
            outputs.append([nums1[idx1],nums2[idx2]])
            write_tables(idx1,idx2,1)
        
        if len(nums1)==0 or len(nums2) == 0:
            return []        
        
        candidates = [(nums1[0]+nums2[0],(0,0))]
        heapq.heapify(candidates)
        outputs = []
        
        while len(outputs) < k and len(candidates) > 0:
            _,(idx1,idx2) = heapq.heappop(candidates)
            push_output_and_write_table(idx1,idx2)
            if idx1+1 < len(nums1) and read_tables(idx1+1,idx2-1) == 1:
                heapq.heappush(candidates, (nums1[idx1+1]+nums2[idx2],(idx1+1,idx2)))
            if idx2+1 < len(nums2) and read_tables(idx1-1,idx2+1) == 1:
                heapq.heappush(candidates, (nums1[idx1]+nums2[idx2+1],(idx1,idx2+1)))
            
        
        return outputs