二进制求和

输入: a = "11", b = "1"
输出: "100"

输入: a = "1010", b = "1011"
输出: "10101"

/**
 * @param {string} a
 * @param {string} b
 * @return {string}
 */
var addBinary = function(a, b) {
    a=a.split('').reverse()
    b=b.split('').reverse()
    while(a.length > b.length){
        b.push('0');
    }
    while(a.length < b.length){
        a.push('0');
    }
    let stacks = [];
    for(let i = 0;i < a.length; i++){
        const s = Number(a[i]) + Number(b[i]) + (stacks[i] ? 1 : 0);
        stacks[i] = s%2;
        if(s>1){
            stacks.push(extra)
        }
    }
    stacks = stacks.reverse().join('');
    return stacks
};