Number of Enclaves

You are given an `m x n` binary matrix `grid`, where `0` represents a sea cell and `1` represents a land cell. A **move** consists of walking from one land cell to another adjacent (**4-directionally**) land cell or walking off the boundary of the `grid`. Return _the number of land cells in_ `grid` _for which we cannot walk off the boundary of the grid in any number of **moves**_.   **Example 1:** **Input:** grid = [[0,0,0,0],[1,0,1,0],[0,1,1,0],[0,0,0,0]] **Output:** 3 **Explanation:** There are three 1s that are enclosed by 0s, and one 1 that is not enclosed because its on the boundary. ``` **Example 2:** **Input:** grid = [[0,1,1,0],[0,0,1,0],[0,0,1,0],[0,0,0,0]] **Output:** 0 **Explanation:** All 1s are either on the boundary or can reach the boundary. ```   **Constraints:** - `m == grid.length` - `n == grid[i].length` - `1 <= m, n <= 500` - `grid[i][j]` is either `0` or `1`.