1020. Number of Enclaves
Medium
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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`.
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