<p>You have <code>k</code> lists of sorted integers in <strong>non-decreasing&nbsp;order</strong>. Find the <b>smallest</b> range that includes at least one number from each of the <code>k</code> lists.</p>

<p>We define the range <code>[a, b]</code> is smaller than range <code>[c, d]</code> if <code>b - a &lt; d - c</code> <strong>or</strong> <code>a &lt; c</code> if <code>b - a == d - c</code>.</p>

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

<pre>
<strong>Input:</strong> nums = [[4,10,15,24,26],[0,9,12,20],[5,18,22,30]]
<strong>Output:</strong> [20,24]
<strong>Explanation: </strong>
List 1: [4, 10, 15, 24,26], 24 is in range [20,24].
List 2: [0, 9, 12, 20], 20 is in range [20,24].
List 3: [5, 18, 22, 30], 22 is in range [20,24].
</pre>

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

<pre>
<strong>Input:</strong> nums = [[1,2,3],[1,2,3],[1,2,3]]
<strong>Output:</strong> [1,1]
</pre>

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

<ul>
	<li><code>nums.length == k</code></li>
	<li><code>1 &lt;= k &lt;= 3500</code></li>
	<li><code>1 &lt;= nums[i].length &lt;= 50</code></li>
	<li><code>-10<sup>5</sup> &lt;= nums[i][j] &lt;= 10<sup>5</sup></code></li>
	<li><code>nums[i]</code>&nbsp;is sorted in <strong>non-decreasing</strong> order.</li>
</ul>

class Solution {
public:
    vector<int> smallestRange(vector<vector<int>>& nums) {
        map<int, vector<int>> buckets;
        unordered_map<int, int> occurances;
        unordered_set<int> window;

        for (int i = 0; i < nums.size(); ++i) {
            for (const int val : nums[i]) {
                vector<int> &bucket = buckets[val];
                if (bucket.empty() || bucket.back() != i) {
                    bucket.push_back(i);
                }
            }
        }
        vector<int> ans = {0, INT_MAX};
        for (auto l = buckets.begin(), r = buckets.begin(); r != buckets.end(); ++r) {
            vector<int> &bucketR = (*r).second;
            for (int i : bucketR) {
                if (++occurances[i] == 1) {
                    window.insert(i);
                }
            }
            while (window.size() == nums.size()) {
                vector<int> &bucketL = (*l).second;
                int first = (*l).first, last = (*r).first;
                if (last - first < ans[1] - ans[0] ||
                    (last - first == ans[1] - ans[0] && first < ans[0])) {
                    ans = {first, last};
                }
                for (int i : bucketL) {
                    if (--occurances[i] == 0) {
                        window.erase(i);
                    }
                }
                ++l;
            }
        }
        return ans;
    }
};
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