algorithm code trick

1.二维数组上下左右移动时判断是否越界

https://www.cser.club/leetcode/#/leetcode/407.TrappingRainWaterII

// BFS上下左右
final int[][] neighbors = new int[][]{{-1, 0}, {1, 0}, {0, -1}, {0, 1}};

while (!minHeap.isEmpty()) {
    // 当前墙的高度
    int tmpWall = minHeap.peek().height;
    while (minHeap.peek() != null && minHeap.peek().height <= tmpWall) {
        Cell tmpCell = minHeap.poll();
        // BFS上下左右
        for (int i = 0; i < 4; i++) {
            int tmpRow = tmpCell.row + neighbors[i][0];
            int tmpCol = tmpCell.col + neighbors[i][1];
            // 判断上下左右是否在范围内，因为最外面一层肯定没用，所以判断条件随便写
            if (tmpRow > 0 && tmpRow < ROWS && tmpCol > 0 && tmpCol < COLS && !visited[tmpRow][tmpCol]) {
                visited[tmpRow][tmpCol] = true;
                minHeap.add(new Cell(tmpRow, tmpCol, heightMap[tmpRow][tmpCol]));
                res += Math.max(0, tmpWall - heightMap[tmpRow][tmpCol]); // 如果比墙矮，则可以存水。
            }
        }
    }
}