unreasonably effective

link Brute force For each day we can scan into the future and pick the first day with a higher temperature. Time: . Stack If the temperatures are sorted in non-increasing order, no days will have a warmer day in the future. So, if any day has a future warmer day, there must be two…

link Queue preserves the write order. Stack reverses the write order. If we reverse write order twice, we get back the original order. So, we can pass the push()‘s through the two stacks (left and right) in sequence: . Here would be the push()and would be peek() or pop(). Amortized, each push(), peek(), pop() and…

link We want to flatten the list lazily, so we use a stack with the top as the next element to return. If hasNext() returns true, the next() must return an integer. So, hasNext() must guarantee that there is an integer to return. We could not simply use len() as the signal for hasNext() because,…

link From pairs of consecutive logs we find exclusive time piecemeal and attribute to the correct function. Exclusive cpu time A log represents two timestamps: (1) end timestamp of the previous function and (2) start timestamp of the next function. If log timestamp is , then: From a pair of consecutive logs: and , we…

link Brute force Try all possibilities. For every opening parenthesis we can take it or remove it. For every closing parenthesis, we can remove it or if close count is less than open count, we can take it. Time: , space: . Greedy With a stack we try to match (or pop) an open as…

link Stack We can consider an adjacent duplicate pair like “aa” or “cc” as “()”. Removal of such a pair may produce more duplicate pairs only if there is nesting like: “(())”. For example: “zaaz” or “poop”. So, we use a stack to remove the nested duplicate pairs. Time: , space: same. Two pointers If…

link We want to process the string as a sequence of tokens where each token is an operator or an opening (or closing) parenthesis or a multi-digit number. So, we use a stack to store tokens as we process the string from left-to-right. All binary operators, no parentheses If we only had binary operators and…