Remove Duplicate Letters for Lexicographically Smallest Result

Given a string, remove duplicate letters so each character appears exactly once and the final string is lexicographically smallest among all valid results. The round explicitly included the two standard examples `bcabc -> abc` and `cbacdcbc -> acdb`, with a monotonic-stack solution expected. Requirements Given a string s, remove duplicate letters so that every letter appears once and only once. Return the lexicographically smallest result among all valid strings. Examples Input: s = "bcabc" Output: "abc" Input: s = "cbacdcbc" Output: "acdb" Notes The interviewer accepted the monotonic-stack framing: track whether a character is already in the stack, and only pop a larger top character when it appears again later. The round starts with roughly 30 minutes of project discussion plus DB / Redis / Kafka concepts, leaving about 30 minutes for this coding task. Common bug: popping a character whose last occurrence has already passed makes it impossible to include every distinct letter exactly once. Preparation Drill the monotonic-stack template with last_index, in_stack, and a mutable result stack. Trace both provided examples by hand; they expose the two key cases: safe pop versus must-keep. Prepare a one-minute explanation of why the stack remains lexicographically minimal without losing required characters.