Approach

A building at (x, y) is considered covered if there exists another building on both sides vertically in the same column and horizontally in the same row.

For each x, track the minimum and maximum y, and for each y, track the minimum and maximum x. A building is covered if it lies strictly within both its column and row ranges.

/**
 * @param {number} n
 * @param {number[][]} buildings
 * @return {number}
 */
var countCoveredBuildings = function(n, buildings) {
    let mapX = {};
    let mapY = {};

    for (let [x, y] of buildings) {
        if (!mapX[x]) mapX[x] = [y, y];
        else {
            let [minY, maxY] = mapX[x];
            mapX[x] = [Math.min(minY, y), Math.max(maxY, y)];
        }

        if (!mapY[y]) mapY[y] = [x, x];
        else {
            let [minX, maxX] = mapY[y];
            mapY[y] = [Math.min(minX, x), Math.max(maxX, x)];
        }
    }

    let count = 0;

    for (let [x, y] of buildings) {
        let [minY, maxY] = mapX[x];
        let [minX, maxX] = mapY[y];

        if (x > minX && x < maxX && y > minY && y < maxY) {
            count++;
        }
    }

    return count;
};