题目：

题解：

class Solution {
public:
    int trapRainWater(vector<vector<int>>& heightMap) {
        int m = heightMap.size(), n = heightMap[0].size();
        int maxHeight = 0;
        int dirs[] = {-1, 0, 1, 0, -1};

        for (int i = 0; i < m; ++i) {
            maxHeight = max(maxHeight, *max_element(heightMap[i].begin(), heightMap[i].end()));
        }
        vector<vector<int>> water(m, vector<int>(n, maxHeight));
        queue<pair<int,int>> qu;
        for (int i = 0; i < m; ++i) {
            for (int j = 0; j < n; ++j) {
                if (i == 0 || i == m - 1 || j == 0 || j == n - 1) {
                    if (water[i][j] > heightMap[i][j]) {
                        water[i][j] = heightMap[i][j];
                        qu.push(make_pair(i, j));
                    }
                }
            }
        }        
        while (!qu.empty()) {
            int x = qu.front().first, y = qu.front().second;
            qu.pop();
            for (int i = 0; i < 4; ++i) {
                int nx = x + dirs[i], ny = y + dirs[i + 1];
                if (nx < 0 || nx >= m || ny < 0 || ny >= n) {
                    continue;
                }
                if (water[x][y] < water[nx][ny] && water[nx][ny] > heightMap[nx][ny]) {
                    water[nx][ny] = max(water[x][y], heightMap[nx][ny]);
                    qu.push(make_pair(nx, ny));
                }
            }
        }

        int res = 0;
        for (int i = 0; i < m; ++i) {
            for (int j = 0; j < n; ++j) {
                res += water[i][j] - heightMap[i][j];
            }
        }
        return res;
    }
};