# Question

Formatted question description: https://leetcode.ca/all/228.html

 228. Summary Ranges

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.

Example 2:

Input:  [0,2,3,4,6,8,9]
Output: ["0","2->4","6","8->9"]
Explanation: 2,3,4 form a continuous range; 8,9 form a continuous range.

@tag-array



# Algorithm

Traverse the array once and check whether the next number is increasing each time

• If yes, continue to traverse down
• If not, we have to judge whether it is a number or a sequence at this time,
• A number is directly stored in the result
• The sequence should be stored in the first and last numbers and the arrow "->".

We need two variables i and j, where i is the position of the starting number of the continuous sequence, j is the length of the continuous sequence,

• when j is 1, it means there is only one number,
• if it is greater than 1, it is a continuous sequence.

# Code

Java

• import java.util.ArrayList;
import java.util.List;

public class Summary_Ranges {

class Solution {
public List<String> summaryRanges(int[] nums) {
List<String> result = new ArrayList<>();
int left = 0, n = nums.length;
while (left < n) {
int right = 1;
while (left + right < n && (long)nums[left + right] - nums[left] == right) {
++right;
}
right <= 1 ? // note: right here is already after ++ in above loop. so it's actually checking if it's a single digit
String.valueOf(nums[left])
: String.valueOf(nums[left]) + "->" + String.valueOf(nums[left + right - 1])); // also here, -1 because already done ++
left += right;
}

return result;

}
}
}

• // OJ: https://leetcode.com/problems/summary-ranges/
// Time: O(N)
// Space: O(N)
class Solution {
public:
vector<string> summaryRanges(vector<int>& A) {
vector<string> ans;
int start = 0;
for (int i = 0, N = A.size(); i < N; ++i) {
if (i + 1 < N && A[i + 1] == A[i] + 1) continue;
ans.push_back(to_string(A[start]) + (i == start ? "" : ("->" + to_string(A[i]))));
start = i + 1;
}
return ans;
}
};

• class Solution(object):
def summaryRanges(self, nums):
"""
:type nums: List[int]
:rtype: List[str]
"""

def outputRange(start, end):
if start == end:
return str(start)
return "{}->{}".format(start, end)

if not nums:
return []
ans = []
start = 0
for i in range(0, len(nums) - 1):
if nums[i] + 1 != nums[i + 1]:
ans.append(outputRange(nums[start], nums[i]))
start = i + 1
ans.append(outputRange(nums[start], nums[-1]))
return ans