Along a long library corridor, there is a line of seats and decorative plants. You are given a 0-indexed string corridor of length n consisting of letters 'S' and 'P' where each 'S' represents a seat and each 'P' represents a plant. One room divider

numberOfWays.js

/**
 * Calculates the number of ways to divide a corridor into valid sections.
 * Each section must contain exactly two 'S' (seats).
 * 'P' (plants) between pairs of seats determine possible partitions.
 *
 * @param {string} corridor - A string consisting of 'S' (seats) and 'P' (plants).
 * @return {number} - The number of valid ways to divide the corridor.
 */
var numberOfWays = function(corridor) {
    const MOD = 1e9 + 7;  // Large prime modulus to prevent overflow
    let product = 1;      // Final result (multiplicative accumulation of choices)
    let seatCount = 0;    // Tracks how many seats ('S') have been seen
    let plantCount = 0;   // Tracks plants ('P') between pairs of seats

    // Traverse the corridor string
    for (let i = 0; i < corridor.length; i++) {
        const char = corridor[i];

        // Count seats
        if (char === 'S') {
            seatCount++;
        }

        // Count plants only when we are between exactly two seats
        if (seatCount === 2 && char === 'P') {
            plantCount++;
        }

        // When we encounter the 3rd seat:
        // - We close off the previous section (which had 2 seats).
        // - The number of ways to partition depends on how many plants were in between.
        // - Reset plantCount and continue with the next section.
        if (seatCount === 3) {
            product = (product * (plantCount + 1)) % MOD;
            plantCount = 0;
            seatCount = 1;  // Carry over the last seat to start the next section
        }
    }

    // If fewer than 2 seats exist, no valid partition is possible
    if (seatCount < 2) {
        return 0;
    }

    return product;
};