Given a sorted integer array where the range of elements are [lower, upper] inclusive, return its missing ranges.

For example, given [0, 1, 3, 50, 75], lower = 0 and upper = 99, return ["2", "4->49", "51->74", "76->99"].

思路分析：这题基本就是考察双指针的应用，从lower-1开始，遍历nums数组找出missing range，可以利用pre和cur一前一后两个指针移动实现。注意lower和upper的处理，lower可以比nums中最小的数大，upper可以比nums中最大的数小，所以考虑要周全。

参考Code（Book中的题目）

public List findMissingRanges(int[] nums, int lower, int upper) {        
        List res = new ArrayList();
        int pre = lower - 1;
        for(int i = 0; i < nums.length; i++){
            if(i == nums.length){
                cur = upper + 1;
            } else cur = nums[i];
            if(cur - pre >= 2){
                int missStart = pre + 1;
                int missEnd = cur - 1;
                if(missEnd == missStart) res.add(missEnd);
                else res.add(missStart + "->" + missEnd);
                pre = cur;
            } else {
                pre = cur;
            }
        }
        return res;
    }