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Summary Ranges

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Question

Given a sorted integer array without duplicates, return the summary of its ranges.

Example 1:
Input: [0,1,2,4,5,7]
Output: [“0->2”,”4->5”,”7”]
Explanation: 0,1,2 form a continuous range; 4,5 form a continuous range.

Solution

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/*
@param: int[] nums
@return: List<String>: ranges
Algorithm: 维护一个start,check if nums[i] = nums[i-1]+1,如果不是更新start,add result；
stringBuilder效率更高
*/
public List<String> summaryRanges(int[] nums) {
    List<String> result = new ArrayList<>();
    if (nums.length == 0) {
        return result;
    }
    
    int start = nums[0];
    for (int i = 1; i < nums.length; i++) {
        if (nums[i] == nums[i-1] + 1) {
            continue;
        }else {                
            if (nums[i-1] == start) {
                add(start, start, result);
            }else {
                add(start, nums[i-1], result);
            }                                
            start = nums[i];
        }
    }
    if (nums[nums.length-1] == start) {
        add(start, start, result);
    }else {
        add(start, nums[nums.length-1], result);
    }      
    return result;
}

public void add(int start, int end, List<String> result) {
    StringBuilder sb = new StringBuilder();
    if (end == start) {
        sb.append(start);
        result.add(sb.toString());
    }else {
        sb.append(start);
        sb.append("->");
        sb.append(end);
        result.add(sb.toString());
    }         
}