# 0696. Count Binary Substrings

<https://leetcode.com/problems/count-binary-substrings>

## Description

Give a binary string `s`, return the number of non-empty substrings that have the same number of `0`'s and `1`'s, and all the `0`'s and all the `1`'s in these substrings are grouped consecutively.

Substrings that occur multiple times are counted the number of times they occur.

**Example 1:**

```
**Input:** s = "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.
```

**Example 2:**

```
**Input:** s = "10101"
**Output:** 4
**Explanation:** There are 4 substrings: "10", "01", "10", "01" that have equal number of consecutive 1's and 0's.
```

**Constraints:**

* `1 <= s.length <= 105`
* `s[i]` is either `'0'` or `'1'`.

## ac

```java
class Solution {
    public int countBinarySubstrings(String s) {
        // edge cases

        int res = 0;

        int prev = 0, curr = 1;
        for (int i = 1; i < s.length(); i++) {
            if (s.charAt(i) != s.charAt(i-1)) {
                res += Math.min(prev, curr);
                prev = curr;
                curr = 1;
            } else {
                curr++;
            }
        }
        res += Math.min(prev, curr); // last part

        return res;
    }
}
```


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