Find a Same-Letter Loop in a Grid

### Problem Given a rectangular lowercase-character `grid`, decide whether four-directional moves can form a cycle of at least four cells, all carrying the same character. The path returns to its starting cell, and a move may not immediately reverse the preceding move. Return true if such a cycle exists. ```cpp class Solution { public: bool containsCycle(vector<vector<char>>& grid); }; ``` ### Examples **Example 1:** ```text grid = [["q","q"],["q","q"]] Output: true Explanation: The four perimeter cells form a same-letter cycle. ``` **Example 2:** ```text grid = [["a","a","b"],["c","a","b"]] Output: false Explanation: Each equal-letter connected component is a path or a single cell. ``` ### Constraints - `m == grid.length` and `n == grid[i].length`. - `1 <= m,n <= 500`. - Every cell contains a lowercase English letter. ### Clarifying Questions Only up, down, left, and right moves count. Traversing one edge forward and backward does not form a valid cycle. ```hint Inspect the structure How will you distinguish a valid equal-letter loop from immediately retracing a single edge? ``` ### Reasoning checkpoints - Show that a detected loop obeys the equal-character, four-direction, and minimum-length rules. - Explain why the test neither accepts immediate edge reversal nor misses a valid cycle elsewhere in the grid. ### Optional extensions How would diagonal moves change the minimum possible cycle length?