leetcode 454. 4Sum II

Seawolf

1. Given four lists A, B, C, D of integer values, compute how many tuples (i, j, k, l) there are such that A[i] + B[j] + C[k] + D[l] is zero.
   
2. 
   
3. To make problem a bit easier, all A, B, C, D have same length of N where 0 ≤ N ≤ 500. All integers are in the range of -228 to 228 - 1 and the result is guaranteed to be at most 231 - 1.
   
4. 
   
5. Example:
   
6. 
   
7. Input:
   
8. A = [ 1, 2]
   
9. B = [-2,-1]
   
10. C = [-1, 2]
    
11. D = [ 0, 2]
    
12. 
    
13. Output:
    
14. 2
    
15. 
    
16. Explanation:
    
17. The two tuples are:
    
18. 1. (0, 0, 0, 1) -> A[0] + B[0] + C[0] + D[1] = 1 + (-2) + (-1) + 2 = 0
    
19. 2. (1, 1, 0, 0) -> A[1] + B[1] + C[0] + D[0] = 2 + (-1) + (-1) + 0 = 0

复制代码

1. class Solution {
   
2.     public int fourSumCount(int[] A, int[] B, int[] C, int[] D) {
   
3.         HashMap<Integer,Integer> map = new HashMap<>();
   
4.         for(int i : A){
   
5.             
   
6.             for(int j: B){
   
7.                 map.merge(i+j,1,(new BiFunction<Integer,Integer,Integer>(){public Integer apply(Integer i1, Integer i2){return i1+i2;}}));
   
8.             }
   
9.         }
   
10.         int ret = 0;
    
11.         for(int i : C){
    
12.             
    
13.             for(int j: D){
    
14.                 ret += map.getOrDefault(-i-j,0);
    
15.             }
    
16.         }
    
17.         return ret;
    
18.         
    
19.     }
    
20. }

复制代码