Find the minimum window substring.

This problem can be efficiently solved using the sliding window technique combined with a frequency map (or hash map). The core idea is to maintain a window within the string s and expand it until it contains all characters from string t with the required frequencies. Once a valid window is found, we try to shrink it from the left while maintaining the condition, updating the minimum window found so far. We use a frequency map to keep track of the characters needed from t and another map to track the characters within the current window. The window slides by moving the right pointer to expand and the left pointer to contract.