<p>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 <strong>nice</strong> if it satisfies all of the following conditions:</p>

<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>

<p>Return <em>the number of nice pairs of indices</em>. Since that number can be too large, return it <strong>modulo</strong> <code>10<sup>9</sup> + 7</code>.</p>

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

<pre>
<strong>Input:</strong> nums = [42,11,1,97]
<strong>Output:</strong> 2
<strong>Explanation:</strong> 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>

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

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

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

<ul>
	<li><code>1 &lt;= nums.length &lt;= 10<sup>5</sup></code></li>
	<li><code>0 &lt;= nums[i] &lt;= 10<sup>9</sup></code></li>
</ul>

typedef long long int lli;
class Solution {
public:
    const int mod=1e9+7;
    int rev(int i){
        int ans=0;
        while(i>0){
            int rem=i%10;
            ans=ans*10+rem;
            i=i/10;
        }
        return ans;
    }
    
    int countNicePairs(vector<int>& nums) {
        lli ans=0;
        vector<int> r(nums.size());
        for(int i=0;i<nums.size();i++){
            r[i]=rev(nums[i]);
        }
        unordered_map<int,lli> m;
        for(int i=0;i<nums.size();i++){
            int val=nums[i]-r[i];
            m[val]++;
        }
        for(auto j:m){
            // cout<<j.second<<" ";
            ans=(ans+(lli)(j.second*(j.second-1))/2)%mod;
        }
        return ans;
    }
};
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