This approach revolves around using a stack to efficiently identify and remove the substrings "AB" and "CD" from the given string. By traversing each character of the string:

• If the current character and the top of the stack form "AB" or "CD", pop from the stack as they form a pattern to remove.
• If not, push the current character onto the stack. This keeps future potential patterns intact until further examination.

The characters that remain on the stack are the ones that could not form any removable patterns, thus forming the result string.

This technique uses a two-pointer approach to reduce the string by altering it in place. The core idea is to selectively overwrite positions in the string:

• Incrementally check pairs of characters: if they comprise a known removable substring, skip them both.
• If not, replace the write pointer's position with the read character, then advance the write position after adjustments.

After reaching the end of the string, the length of the string from the start to the write pointer represents the reduced string length.