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统计子串中的唯一字符（找规律）

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大佬的题解

class Solution:def uniqueLetterString(self, s: str) -> int:ans = total = 0last0, last1 = {}, {}for i, c in enumerate(s):total += i - 2 * last0.get(c, -1) + last1.get(c, -1)ans += totallast1[c] = last0.get(c, -1)last0[c] = ireturn ans