leetcode923 -- 3Sum With Multiplicity

# 小郑之家~

### 题目

Given an integer array A, and an integer target, return the number of tuples i, j, k  such that i < j < k and A[i] + A[j] + A[k] == target.

As the answer can be very large, return it modulo 10^9 + 7.

Example 1:

Input: A = [1,1,2,2,3,3,4,4,5,5], target = 8
Output: 20
Explanation:
Enumerating by the values (A[i], A[j], A[k]):
(1, 2, 5) occurs 8 times;
(1, 3, 4) occurs 8 times;
(2, 2, 4) occurs 2 times;
(2, 3, 3) occurs 2 times.
Example 2:

Input: A = [1,1,2,2,2,2], target = 5
Output: 12
Explanation:
A[i] = 1, A[j] = A[k] = 2 occurs 12 times:
We choose one 1 from [1,1] in 2 ways,
and two 2s from [2,2,2,2] in 6 ways.



class Solution(object):
def threeSumMulti(self, A, target):
"""
:type A: List[int]
:type target: int
:rtype: int
"""
res = 0
dic = {}
for x in A:
if x not in dic:
dic[x] = 1
else:
dic[x] += 1

for i, x in dic.items():
for j, y in dic.items():
k = target -i-j
if k not in dic:
continue
if i==j==k:
res += x*(x-1)*(x-2)/6
elif i==j and j!= k:
res += x*(x-1)/2 *dic[k]
elif i<j and j<k:
res += x*y*dic[k]

return res%(10**9+7)