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Intuition\n Describe your first thoughts on how to solve this problem. \n\n# Approach\n Describe your approach to solving the problem. \n\n# Complexity\n- Time 
# Intuition\n<!-- Describe your first thoughts on how to solve this problem. -->\n\n# Approach\n<!-- Describe your approach to solving the problem. -->\n\n# Complexity\n- Time complexity:\n<!-- Add your time complexity here, e.g. $$O(n)$$ -->\n\n- Space complexity:\n<!-- Add your space complexity here, e.g. $$O(n)$$ -->\n\n# Code\n```python3 []\nclass Solution:\n    def validSubstringCount(self, a: str, b: str) -> int:\n        # Initialize frequency arrays for characters in \'b\' and for the window in \'a\'\n        cnt = [0] * 26  # Count of characters in string \'b\'\n        arr = [0] * 26  # Current count of characters in window of string \'a\'\n        x = 0           # Number of characters from \'b\' left to match\n        n = len(a)      # Length of string \'a\'\n        l = 0           # Left pointer of the sliding window\n        res = 0         # Result to store the number of valid substrings\n\n        # Populate \'cnt\' with the frequencies of characters in \'b\'\n        for c in b:\n            cnt[ord(c) - ord(\'a\')] += 1\n            x += 1  # Track total characters to match\n\n        # Sliding window approach\n        for r in range(n):\n            i = ord(a[r]) - ord(\'a\')  # Convert current character in \'a\' to an index\n            arr[i] += 1  # Increase the count of this character in the window\n\n            if arr[i] <= cnt[i]:\n                x -= 1  # Decrease \'x\' if a character from \'b\' is matched\n\n            # Shrink the window from the left when all characters are matched\n            while x == 0:\n                res += n - r  # Add all possible substrings starting from this window\n                i = ord(a[l]) - ord(\'a\')  # Get index for the left pointer character\n                arr[i] -= 1  # Remove the character from the window\n                l += 1  # Move the left pointer to the right\n                if arr[i] < cnt[i]:\n                    x += 1  # Increase \'x\' when a character is unmatched\n\n        return res\n```
['Python3']