//我也试了一下大数相加
function add(a,b){
    if(typeof a !== 'string'||typeof b !== 'string'){
        console.error('require two strings')
        return
    }
    if(isNaN(+a)||isNaN(+b)){
        console.error('require to be normal number string')
        return        
    }
    var min = a.length>b.length?b:a
    var max = a.length<b.length?b:a
    var len = min.length
    var lenmax = max.length
    var upgrade = 0
    if(a.length===b.length){
        min = a;
        max = b;
    }
    var res = []
    while(len--){
        var sum = parseInt(min[len])+parseInt(max[--lenmax])
        if(upgrade){
            sum += upgrade
            upgrade = 0
        }
        if(sum>=10){
            sum -= 10
            upgrade = 1
        }    
        res.unshift(sum)
    }
    res = res.join("")
    if(upgrade){
        if(lenmax-len-1>0){
            var mid = +max.slice(lenmax-len-2,lenmax-len-1)+1
            res = max.slice(0,lenmax-len-2)+mid+res
        }else if(lenmax-len===1){
            res = 1+parseInt(max.slice(0,1))+res
        }
    }else if(a.length!==b.length){
        res = max.slice(0,lenmax-len-1) + res
    }
    return res
}