[LeetCode] 611. Valid Triangle Number

 Problem : https://leetcode.com/problems/valid-triangle-number/

To form a valid triangle, the 3 side lengths must compel with  A + B > C.

Use binary search to find number of C which less than (A + B).

class Solution:
    def triangleNumber(self, nums: List[int]) -> int:
        nums.sort()
        result = 0
        
        for i in range(len(nums)):
            for j in range(i+1, len(nums)-1):
                k = bisect.bisect_left(nums, nums[i] + nums[j], j+1, len(nums))
                result +=  k - (j+1)
        
        return result

Java solution:

class Solution {
    public int triangleNumber(int[] nums) {
        Arrays.sort(nums);
        int result = 0;
        
        for (int i = 0; i < nums.length -1; i++) {
            for (int j = i+1; j < nums.length - 1; j++) {
                int k = lowerBound(nums, nums[i] + nums[j], j+1, nums.length);
                result += k - (j+1);
            }
        }
        
        return result;
    }
    
    /**
    * Return position of the first number >= target
    */
    int lowerBound(int[] nums, int target, int start, int end) {
        int left = start;
        int right = end;
        
        while (left < right) {
            int mid = left + (right - left) / 2;
            
            if (nums[mid] > target)
                right = mid;
            else if (nums[mid] < target)
                left = mid + 1;
            else if (nums[mid] == target)
                right = mid;
        }
        
        return right;
    }
}