leetcode 228. summary ranges

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

Example 1:

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Input: [0,1,2,4,5,7]
Output: ["0->2","4->5","7"]

Example 2:

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Input: [0,2,3,4,6,8,9]
Output: ["0","2->4","6","8->9"]

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class  {
public List<String> summaryRanges(int[] nums) {
List<String> ans = new ArrayList<String>();
if(nums.length == 0) return ans;
StringBuilder ab = new StringBuilder();
boolean isStart = true;
int i=0;
int j=1;
if(nums.length == 1){
ab.append(nums[0]);
ans.add(ab.toString());
ab = null;
return ans;
}
while(i<nums.length && j<nums.length){
if(nums[j]!=nums[i]+1){
ab.append(nums[i]);
ans.add(ab.toString());
ab.delete(0, ab.length());
i=j;
j=j+1;

}
else{
while(j<nums.length && nums[j]==nums[j-1]+1){
j++;
}
if(j > nums.length-1){
ab.append(nums[i] + "->" + nums[j-1]);
ans.add(ab.toString());
ab.delete(0, ab.length());
i = j+1;
j = j+2;
}
else{
ab.append(nums[i] + "->" + nums[j-1]);
ans.add(ab.toString());
ab.delete(0, ab.length());
i = j;
j = j+1;
}

}

}
if(j == nums.length-1){
ab.append(nums[j]);
ans.add(ab.toString());
}
if(i == nums.length-1){
ab.append(nums[i]);
ans.add(ab.toString());
}
return ans;
}
}

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List<String> list=new ArrayList();
if(nums.length==1){
list.add(nums[0]+"");
return list;
}
for(int i=0;i<nums.length;i++){
int a=nums[i];
while(i+1<nums.length&&(nums[i+1]-nums[i])==1){
i++;
}
if(a!=nums[i]){
list.add(a+"->"+nums[i]);
}else{
list.add(a+"");
}
}
return list;