Stack, Reverse Polish Notation

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32:T4f4, - This problem can be solved using backtracking. The goal is to build a valid string of parentheses step by step. - The state for our recursive function can be the number of open parentheses used (`openN`) and the number of closed parentheses used (`closedN`). - We can add an opening parenthesis '(' only if we haven't exceeded the total number of pairs, `n`. - We can add a closing parenthesis ')' only if the number of closed parentheses is less than the number of open parentheses. This rule ensures the string remains well-formed at all times. - **Algorithm**: 1. Start a recursive function `backtrack(openN, closedN)`. 2. **Base Case**: If `openN == closedN == n`, it means we have formed a complete, well-formed string. Add the current string to the result list and return. 3. **Recursive Step (Choice 1 - Add '(')**: If `openN < n`, we can add an opening parenthesis. Append '(' to our current path/stack, and recurse: `backtrack(openN + 1, closedN)`. After the call returns, pop '(' to backtrack. 4. **Recursive Step (Choice 2 - Add ')')**: If `closedN < openN`, we can add a closing parenthesis. Append ')' to the path, and recurse: `backtrack(openN, closedN + 1)`. After the call, pop ')' to backtrack. - The initial call will be `backtrack(0, 0)`.33:T4a9, - This problem asks for the "next warmer temperature", which is a classic use case for a **monotonic stack**. - We want to maintain a stack of temperatures that are in a monotonically decreasing order. - **Algorithm**: 1. Initialize a result array with all zeros. 2. Initialize an empty stack. The stack will store pairs of `[temperature, index]`. 3. Iterate through the input `temperatures` array with its index `i` and temperature `t`. 4. For each temperature `t`, check the stack: while the stack is not empty and the

Level Order Traversal (#102)

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