Find the largest rectangular area that can fit within a histogram defined by an array of bar heights. You are given an array representing the height of bars, where each bar has a width of 1.

The problem involves decoding a string that follows a specific encoding rule: k[encoded_string], where the encoded_string inside the square brackets is repeated exactly k times. Your task is to decode the string and return the decoded string.

Design and implement a Least Frequently Used (LFU) cache. The cache should efficiently store key-value pairs, evicting the least frequently used items when it reaches capacity.