1575 articles
Find the area of the largest rectangle in a histogram. Monotonic increasing stack: when a shorter bar is found, pop and compute rectangle area using popped height and current width.
Calculate water trapped between bars. Two-pointer approach: maintain max_left and max_right; water at each position = min(max_left, max_right) - height.
Remove duplicates to get smallest lexicographic subsequence with all chars. Greedy: use monotonic stack, only pop if char appears later.
Remove k digits to get smallest number. Monotonic increasing stack: pop larger digits when they appear before a smaller one. Remove remaining from end.
Sum of minimums of all subarrays. For each element, count how many subarrays have it as minimum using previous/next smaller element positions.