LeetCode 163 Missing Ranges

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"].

Diffculty
Medium

Similar Problems
[LeetCode 106] Construct Binary Tree from Inorder and Postorder Traversal

Analysis

参考： http://www.cnblogs.com/airwindow/p/4796742.html

This kind problem is easy, it just test your proramming skills. Basic idea: According to the problem, the gap between two numbers: nums[i], nums[i+1] should be recorded. There could be following situation:

------------------------------------------------------------------------------------------------
case 1: no gap (nums[i] + 1 == nums[i+1])
We need to record nothing.
------------------------------------------------------------------------------------------------
case 2: the gap's length is 1. (nums[i] + 2 = nums[i+1])
We need to record one number. nums[i]+1.
------------------------------------------------------------------------------------------------
case 3: the gap's length is larger than 1. (nums[i] + 2 < nums[i+1])
We need to record the range. [nums[i]+1, nums[i+1]-1].


Write in this way is a little ugly, what's more we have lower and upper must be included. we could use two variables for this purpose: (to compute the gap between nums[i-1] and nums[i])
------------------------------------------------------------------------------------------------
after: the number after nums[i-1]
pre: the number before nums[i]
------------------------------------------------------------------------------------------------
if (pre == after) 
    ret.add(pre + "");
else if (pre > after)
    ret.add(after + "->" + pre);
after = nums[i] + 1;

Skill: To involve the lower and upper, we could assign "after" with "lower" before scanning the nums. And assign "pre" with "upper" after the scan.

Solutions

int after = lower;
int pre;
for (int i = 0; i < nums.length; i++) {
    ...
}

pre = upper;
if (pre == after) 
    ret.add(pre + "");
else if (pre > after)
    ret.add(after + "->" + pre);

public class Solution {
    public List<String> findMissingRanges(int[] nums,int lower,int upper){
        List<String> res=new ArrayList<String>();
        for(int n:nums){
            int justBelow=n-1;
            if(lower==justBelow) res.add(lower+"");
            else if(lower<justBelow) res.add(lower+"->"+justBelow);
            lower=n+1;
        }
        if(lower==upper) res.add(lower+"");
        else if(lower<upper) res.add(lower+"->"+upper);
        return res;
    }
}

Reference