count-nice-pairs-in-an-array

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Algorithms

You are given an array <code>nums</code> that consists of non-negative integers. Let us define <code>rev(x)</code> as the reverse of the non-negative integer <code>x</code>. For example, <code>rev(123) = 321</code>, and <code>rev(120) = 21</code>. A pair of indices <code>(i, j)</code> is nice if it satisfies all of the following conditions:

<ul>
	<li><code>0 &lt;= i &lt; j &lt; nums.length</code></li>
	<li><code>nums[i] + rev(nums[j]) == nums[j] + rev(nums[i])</code></li>
</ul>

Return the number of nice pairs of indices. Since that number can be too large, return it modulo <code>109 + 7</code>.

&nbsp;
Example 1:

<pre>
Input: nums = [42,11,1,97]
Output: 2
Explanation: The two pairs are:
 - (0,3) : 42 + rev(97) = 42 + 79 = 121, 97 + rev(42) = 97 + 24 = 121.
 - (1,2) : 11 + rev(1) = 11 + 1 = 12, 1 + rev(11) = 1 + 11 = 12.
</pre>

Example 2:

<pre>
Input: nums = [13,10,35,24,76]
Output: 4
</pre>

&nbsp;
Constraints:

<ul>
	<li><code>1 &lt;= nums.length &lt;= 105</code></li>
	<li><code>0 &lt;= nums[i] &lt;= 109</code></li>
</ul>