Valid Triangle Number
<p>Given an integer array <code>nums</code>, return <em>the number of triplets chosen from the array that can make triangles if we take them as side lengths of a triangle</em>.</p>

<p>&nbsp;</p>
<p><strong class="example">Example 1:</strong></p>

<pre>
<strong>Input:</strong> nums = [2,2,3,4]
<strong>Output:</strong> 3
<strong>Explanation:</strong> Valid combinations are: 
2,3,4 (using the first 2)
2,3,4 (using the second 2)
2,2,3
</pre>

<p><strong class="example">Example 2:</strong></p>

<pre>
<strong>Input:</strong> nums = [4,2,3,4]
<strong>Output:</strong> 4
</pre>

<p>&nbsp;</p>
<p><strong>Constraints:</strong></p>

<ul>
	<li><code>1 &lt;= nums.length &lt;= 1000</code></li>
	<li><code>0 &lt;= nums[i] &lt;= 1000</code></li>
</ul>
Greedy; Array; Two Pointers; Binary Search; Sorting
function triangleNumber(nums: number[]): number {
    nums.sort((a, b) => a - b);
    let n = nums.length;
    let ans = 0;
    for (let i = n - 1; i >= 2; i--) {
        let left = 0,
            right = i - 1;
        while (left < right) {
            if (nums[left] + nums[right] > nums[i]) {
                ans += right - left;
                right--;
            } else {
                left++;
            }
        }
    }
    return ans;
}
