题解 | 谐距下标对

import sys
from collections import Counter
n=int(input())
a=list(map(int,sys.stdin.readline().split()))
count=0
#aj-ai=j-i  -> aj-j=ai-i
ai_i=list()#只需记录ai-i然后将相等值进行排列组合就行( n*（n-1）)/2
for i in range(n):
    ai_i.append(a[i]-i)
#记录ai-i出现相同值的个数
count = Counter(ai_i)
res=0
for key,val in count.items():
    res+=(val*(val-1))//2
print(res)