a, b, a and are triangle sides when a + b > c, a+c > b, and b+c > a. If a, b, c in ascending order then only thing needs to check is a+b < c.

Sorting array helps reduce number of checks and can use 2 pointers. Same as 3sum problem.

lo and hi pointer only visits nums elements once. For each lo increasing, hi doesn't need to be reset because if nums[i] + nums[lo] < nums[hi] then of course nums[i] + nums[lo+1] < nums[hi]. So when increase lo, hi should try to go further to the end of array.

```
public int triangleNumber(int[] nums) {
if (nums.length < 3)
return 0;
int count = 0;
Arrays.sort(nums);
for (int i = 0; i < nums.length - 2; i ++){
int lo = i + 1;
int hi = lo;
while(lo < nums.length - 1){
while(hi + 1 < nums.length && nums[hi + 1] < nums[i] + nums[lo])
hi++;
if (hi - lo > 0)
count += hi - lo;
lo++;
}
}
return count;
}
```