Solve Grid Path and Unique Substring

You are given two independent coding problems. For each problem, provide an implementation and analyze the time and space complexity. ### Problem 1: Find a string in a grid Given an `m x n` grid of characters and a target string `word`, determine whether `word` can be formed by a path in the grid. Rules: - The path may start from any cell. - Each next character must come from a horizontally or vertically adjacent cell. - A grid cell may be used at most once in the same path. - Return `true` if such a path exists; otherwise return `false`. Example: ```text grid = [ ['A','B','C','E'], ['S','F','C','S'], ['A','D','E','E'] ] word = "ABCCED" Output: true ``` ### Problem 2: Longest substring with all unique characters Given a string `s`, return the length of the longest contiguous substring that contains no repeated characters. Example: ```text s = "abcabcbb" Output: 3 Explanation: One longest valid substring is "abc". ``` After solving it with the common linear-time approach, also describe or implement an alternative linear-time solution that does **not** explicitly maintain a sliding window by repeatedly moving a left boundary pointer. For example, you may maintain the last index of each character and a dynamic programming value representing the length of the longest valid substring ending at the current position.