题解 | #abb#

n = int(input())
if n < 3: 
    print(0)
else:
    s = input()
    dp = [{} for i in range(n)] # dp[i]记录i之后各个字母出现的次数
    ans = 0
    for i in range(n-2, -1, -1):
        dp[i] = dp[i + 1].copy()
        key = s[i + 1]
        val = dp[i+1].get(key, 0) + 1
        dp[i][key] = val 

    for i in range(n - 3, -1, -1):
        for key, val in dp[i].items(): # 和字母i不同且在之后出现超过两次
            if key != s[i] and val >= 2:
                ans += val * (val - 1) // 2
    print(ans)