汇总区间

function summaryRanges(nums) {
    let pre = nums[0],next = nums[0],arr = [];
    if(nums.length == 0){
        return arr
    }
    for(let i = 1;i < nums.length;i++){
        let item = nums[i]
        if(item - next <= 1){
            next = item
        }else{
            if(pre == next){
                arr.push(pre+‘‘)
            }else{
                arr.push(pre +‘->‘+next)
            }
            pre = item
            next = item
        }
    }
    if(pre == next){
        arr.push(pre+‘‘)
    }else{
        arr.push(pre +‘->‘+next)
    }
    return arr
}