14.重复的DNA序列

var findRepeatedDnaSequences = function(s) {
    let seenTimes = {} //用来存放 10个长度的字符 及其出现的次数（键值对形式）
    let res = []   //存放结果
    let i = 0      //指针
    while(i+10<=s.length){
        let sequence = s.slice(i,i+10) //截取十位
        seenTimes[sequence] = seenTimes[sequence] + 1 || 1  //第一次存入seenTimes中时
                                                            //seenTimes[sequence]为undefined
                                                            //undefined+1 = NaN  则会选择后者 1
        if(seenTimes[sequence] === 2){   //此处不写大于等于2  因为只需存入一次 
            res.push(sequence)
        }
        i++
    }
    return res
};