778-2

class Solution {
public:
    bool dfs(vector<vector<int>> &grid, int t, int r, int c) {
        int n = grid.size();
        if (r < 0 || r >= n ||
            c < 0 || c >= n ||
            grid[r][c] > t || grid[r][c] < 0)
            return false;

        if (r == n-1 && c == n-1)
            return true;

        grid[r][c] = -1;
        int dir[] = {0, 1, 0, -1, 0};
        for (int i = 0; i < 4; ++i) {
            int x = r+dir[i], y = c+dir[i+1];
            if (dfs(grid, t, x, y))
                return true;
        }
        return false;
    }

    int swimInWater(vector<vector<int>>& grid) {
        int n = grid.size(), l = 0, r = n*n-1;
        while (l < r) {
            int m = (r-l)/2+l;
            vector<vector<int>> tmp = grid;
            if (dfs(tmp, m, 0, 0)) {
                r = m;
            } else {
                l = m+1;
            }
        }
        return r;
    }
};