Arrays & Strings II

• Set membership → duplicates in O(n)
• Two pointers → stable element movement
• Sliding window → longest unique substring
• Stack → validate parentheses
• Monotonic stack → next warmer day
• Monotonic deque → sliding window maximum
• Binary search → insertion position
• Sort + sweep → merge intervals

Two pointers

• Track write index for non‑zero elements
• Fill remaining with zeros

Sliding window

• Maintain a map of last‑seen indices
• Move left pointer when you see duplicates

Monotonic stack

• Stack holds indices with decreasing values
• Pop while current is greater

Monotonic deque

• Deque stores candidate indices
• Front is always the window max
• Pop back while smaller than current

Binary search

• Maintain low/high bounds
• Answer is low after the loop

Sort + sweep

• Sort intervals by start
• Merge when next.start <= current.end