Given a string s consisting only of characters a, b and c. Return the number of substrings containing at least one occurrence of all these characters a, b and c.

numberOfSubstrings.js

/**
 * @param {string} s - Input string containing characters 'a', 'b', and 'c'
 * @return {number} - Number of substrings containing at least one 'a', 'b', and 'c'
 */
var numberOfSubstrings = function(s) {
    let count = 0; // To keep track of the number of unique characters 'a', 'b', and 'c' in the current window
    let map = {'a': 0, 'b': 0, 'c': 0}; // Map to store the count of each character 'a', 'b', and 'c'
    let res = 0; // To store the result (number of substrings)
    let l = 0; // Left pointer for the sliding window

    // Loop through each character in the string with the right pointer
    for (let r = 0; r < s.length; r++) {
        if (s[r] in map) {
            // If the character at position 'r' is in the map, increment its count
            if (map[s[r]] === 0) count++;
            map[s[r]]++;
        }

        // If all three characters 'a', 'b', and 'c' are in the current window
        while (count === 3) {
            // Add the number of substrings ending at position 'r'
            res += s.length - r;
            map[s[l]]--;
            if (map[s[l]] === 0) count--; // Decrement the count if a character count reaches zero
            l++; // Move the left pointer to the right
        }
    }

    return res;
};