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
};