Added Solutions to Leetcode 696

Author: shreyash3087

dsa-problems/leetcode-problems/0600-0699.md

@@ -591,7 +591,7 @@ export const problems = [
         "problemName": "696. Count Binary Substrings",
         "difficulty": "Easy",
         "leetCodeLink": "https://leetcode.com/problems/count-binary-substrings",
-        "solutionLink": "#"
+        "solutionLink": "/dsa-solutions/lc-solutions/0600-0699/count-binary-substrings"
     },
     {
         "problemName": "697. Degree of an Array",

dsa-solutions/lc-solutions/0600-0699/0696-count-binary -ubstrings.md

@@ -0,0 +1,184 @@
+---
+id: count-binary-substrings
+title: Count Binary Substrings
+sidebar_label: 0696 - Count Binary Substrings
+tags:
+  - String
+  - Two Pointers
+  - Sliding Window
+description: "This is a solution to the Count Binary Substrings problem on LeetCode."
+---
+
+## Problem Description
+
+Given 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.
+
+### Examples
+
+**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
+
+- $1 <= s.length <= 10^5$
+- `s[i]` is either `'0'` or `'1'`.
+
+## Solution for Count Binary Substrings
+
+### Approach 1: Group By Character
+#### Intuition
+
+We can convert the string `s` into an array groups that represents the length of same-character contiguous blocks within the string. For example, if `s = "110001111000000"`, then `groups = [2, 3, 4, 6]`.
+
+For every binary string of the form `'0' * k + '1' * k` or `'1' * k + '0' * k`, the middle of this string must occur between two groups.
+
+Let's try to count the number of valid binary strings between `groups[i]` and `groups[i+1]`. If we have `groups[i] = 2, groups[i+1] = 3`, then it represents either "00111" or "11000". We clearly can make `min(groups[i], groups[i+1])` valid binary strings within this string. Because the binary digits to the left or right of this string must change at the boundary, our answer can never be larger.
+
+#### Algorithm
+
+Let's create `groups` as defined above. The first element of `s` belongs in its own group. From then on, each element either doesn't match the previous element, so that it starts a new group of size 1, or it does match, so that the size of the most recent group increases by 1.
+
+Afterward, we will take the sum of `min(groups[i-1], groups[i])`.
+
+## Code in Different Languages
+
+<Tabs>
+<TabItem value="java" label="Java">
+  <SolutionAuthor name="@Shreyash3087"/>
+
+```java
+class Solution {
+    public int countBinarySubstrings(String s) {
+        int[] groups = new int[s.length()];
+        int t = 0;
+        groups[0] = 1;
+        for (int i = 1; i < s.length(); i++) {
+            if (s.charAt(i-1) != s.charAt(i)) {
+                groups[++t] = 1;
+            } else {
+                groups[t]++;
+            }
+        }
+
+        int ans = 0;
+        for (int i = 1; i <= t; i++) {
+            ans += Math.min(groups[i-1], groups[i]);
+        }
+        return ans;
+    }
+}
+```
+
+</TabItem>
+<TabItem value="python" label="Python">
+  <SolutionAuthor name="@Shreyash3087"/>
+
+```python
+class Solution(object):
+    def countBinarySubstrings(self, s):
+        groups = [1]
+        for i in xrange(1, len(s)):
+            if s[i-1] != s[i]:
+                groups.append(1)
+            else:
+                groups[-1] += 1
+
+        ans = 0
+        for i in xrange(1, len(groups)):
+            ans += min(groups[i-1], groups[i])
+        return ans
+```
+</TabItem>
+</Tabs>
+
+## Complexity Analysis
+
+### Time Complexity: $O(N)$
+
+> **Reason**:  where N is the length of s. Every loop is through $O(N)$ items with $O(1)$ work inside the for-block.
+
+### Space Complexity: $O(N)$
+
+> **Reason**:  the space used by `groups`.
+
+### Approach 2: Linear Scan
+#### Intuition and Algorithm
+
+We can amend our Approach #1 to calculate the answer on the fly. Instead of storing `groups`, we will remember only `prev = groups[-2]` and `cur = groups[-1]`. Then, the answer is the sum of `min(prev, cur)` over each different final `(prev, cur)` we see.
+
+## Code in Different Languages
+
+<Tabs>
+<TabItem value="java" label="Java">
+  <SolutionAuthor name="@Shreyash3087"/>
+
+```java
+class Solution {
+    public int countBinarySubstrings(String s) {
+        int ans = 0, prev = 0, cur = 1;
+        for (int i = 1; i < s.length(); i++) {
+            if (s.charAt(i-1) != s.charAt(i)) {
+                ans += Math.min(prev, cur);
+                prev = cur;
+                cur = 1;
+            } else {
+                cur++;
+            }
+        }
+        return ans + Math.min(prev, cur);
+    }
+}
+```
+
+</TabItem>
+<TabItem value="python" label="Python">
+  <SolutionAuthor name="@Shreyash3087"/>
+
+```python
+class Solution(object):
+    def countBinarySubstrings(self, s):
+        ans, prev, cur = 0, 0, 1
+        for i in xrange(1, len(s)):
+            if s[i-1] != s[i]:
+                ans += min(prev, cur)
+                prev, cur = cur, 1
+            else:
+                cur += 1
+
+        return ans + min(prev, cur)
+```
+</TabItem>
+</Tabs>
+
+## Complexity Analysis
+
+### Time Complexity: $O(N)$
+
+> **Reason**:  where N is the length of s. Every loop is through $O(N)$ items with $O(1)$ work inside the for-block.
+
+### Space Complexity: $O(1)$
+
+> **Reason**: the space used by `prev`, `cur`, and `ans`.
+
+## References
+
+- **LeetCode Problem**: [Count Binary Substrings](https://leetcode.com/problems/count-binary-substrings/description/)
+
+- **Solution Link**: [Count Binary Substrings](https://leetcode.com/problems/count-binary-substrings/solutions/)