Given a binary string s without leading zeros, return true if s contains at most one contiguous segment of ones. Otherwise, return false.

checkOnesSegment.js

/**
 * @param {string} s
 * @return {boolean}
 */
var checkOnesSegment = function(s) {
    // This flag will turn true once we hit the first '0'
    // after the initial block of ones.
    let seenZero = false;

    // Loop through each character in the string
    for (let char of s) {

        if (char === '0') {
            // We've reached the end of the ones segment
            seenZero = true;
        } else {
            // char === '1'

            // If we've already seen a zero and now see a '1',
            // that means a second segment of ones has started.
            if (seenZero) {
                return false;
            }
        }
    }

    // If we never encountered a second segment of ones,
    // the string is valid.
    return true;
};