leetcode 696

696. Count Binary Substrings

Input: "00110011"
Output: 6
Explanation: There are 6 substrings that have equal number of consecutive 1's and 0's: "0011", "01", "1100", "10", "0011", and "01".

Notice that some of these substrings repeat and are counted the number of times they occur.

Also, "00110011" is not a valid substring because all the 0's (and 1's) are not grouped together.

First, I count the number of  or  grouped consecutively.
For example "" will be [, , , ].

Second, for any possible substrings with  and  grouped consecutively, the number of valid substring will be the minimum number of  and .
For example "", will be min(, ) = , ("", "", "")

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 int countBinarySubstrings(string s) {
        int cur = , pre = , res = ;
        for (int i = ; i < s.size(); i++) {
            if (s[i] == s[i - ]) cur++;
            else {
                res += min(cur, pre);  //主要是这里的技巧，比如"0001111", will be min(3, 4) = 3, ("01", "0011", "000111")
                pre = cur;
                cur = ;
            }
        }
        return res + min(cur, pre);
    }