You have k lists of sorted integers in ascending order. Find the smallest range that includes at least one number from each of the k lists.

We define the range [a,b] is smaller than range [c,d] if b-a < d-c or a < c if b-a == d-c.

Example 1:

Input:[[4,10,15,24,26], [0,9,12,20], [5,18,22,30]]
Output: [20,24]
Explanation: 
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].

Note:

1. The given list may contain duplicates, so ascending order means >= here.
2. 1 <= k <= 3500
3. -105 <= value of elements <= 105.
4. For Java users, please note that the input type has been changed to List<List<Integer>>. And after you reset the code template, you'll see this point.

Approach #1: C++. [Using pointers]

class Solution {
public:
    vector<int> smallestRange(vector<vector<int>>& nums) {
        int size = nums.size();
        vector<int> next(size, 0);
        int minx = 0, miny = INT_MAX;
        bool flag = true;
        for (int i = 0; i < size && flag; ++i) {
            for (int j = 0; j < nums[i].size() && flag; ++j) {
                int min_i = 0, max_i = 0;
                for (int k = 0; k < size; ++k) {
                    if (nums[min_i][next[min_i]] > nums[k][next[k]])
                        min_i = k;
                    if (nums[max_i][next[max_i]] < nums[k][next[k]])
                        max_i = k;
                }
                if (miny - minx > nums[max_i][next[max_i]] - nums[min_i][next[min_i]]) {
                    miny = nums[max_i][next[max_i]];
                    minx = nums[min_i][next[min_i]];
                }
                next[min_i]++;
                if (next[min_i] == nums[min_i].size())
                    flag = false;
            }
        }
        return {minx, miny};
    }
};

Approach #2: Java. [Using pointers and priorityQueue]

class Solution {
    public int[] smallestRange(List<List<Integer>> nums) {
        int minx = 0, miny = Integer.MAX_VALUE, max = Integer.MIN_VALUE;
        int[] next = new int[nums.length];
        boolean flag = true;
        PriorityQueue<Integer> min_queue = new PriorityQueue<Integer>((i, j)->nums[i][next[i]] - nums[j][next[j]]);
        for (int i = 0; i < nums.length; ++i) {
            min_queue.offer(i);
            max = Math.max(max, nums[i][0]);
        }
        for (int i = 0; i < nums.length && flag; ++i) {
            for (int j = 0; j < nums[i].length && flag; ++j) {
                int min_i = min_queue.poll();
                if (miny - minx > max - nums[min_i][next[min_i]]) {
                    minx = nums[min_i][next[min_i]];
                    miny = max;
                }
                next[min_i]++;
                if (next[min_i] == nums[min_i].length) {
                    flag = false;
                    break;
                }
                min_queue.offer(min_i);
                max = Math.max(max, nums[min_i][next[min_i]]);
            }
        }
        return new int[] {minx, miny};
    }
}

I can't understand why it always compile error with this prompt: Line 10: error: cannot find symbol: method length().

This is the C++ version using priority_queue to solve this problem, may be it can understand easily.

#include <vector>
#include <queue>
#include <limits>

using namespace std;

struct Item {
    int val;
    int r;
    int c;
    
    Item(int val, int r, int c): val(val), r(r), c(c) {
    }
};

struct Comp {
    bool operator() (const Item& it1, const Item& it2) {
        return it2.val < it1.val;
    }
};

class Solution {
public:
    vector<int> smallestRange(vector<vector<int>>& nums) {
        priority_queue<Item, vector<Item>, Comp> pq;
        
        int high = numeric_limits<int>::min();
        int n = nums.size();
        for (int i = 0; i < n; ++i) {
            pq.push(Item(nums[i][0], i, 0));
            high = max(high , nums[i][0]);
        }
        int low = pq.top().val;
        
        vector<int> res{low, high};
        
        while (pq.size() == (size_t)n) {
            auto it = pq.top();
            pq.pop();
            
            if ((size_t)it.c + 1 < nums[it.r].size()) {
                pq.push(Item(nums[it.r][it.c + 1], it.r, it.c + 1));
                high = max(high, nums[it.r][it.c + 1]);
                low = pq.top().val;
                if (high - low < res[1] - res[0]) {
                    res[0] = low;
                    res[1] = high;
                }
            }
        }
        
        return res;
    }
};

Approach #3: Python.

class Solution(object):
    def smallestRange(self, A):
        """
        :type nums: List[List[int]]
        :rtype: List[int]
        """
        pq = [(row[0], i, 0) for i, row in enumerate(A)]
        heapq.heapify(pq)
        
        ans = -1e9, 1e9
        
        right = max(row[0] for row in A)
        while pq:
            left, i, j = heapq.heappop(pq)
            if right - left < ans[1] - ans[0]:
                ans = left, right
            if j + 1 == len(A[i]):
                return ans
            v = A[i][j+1]
            right = max(right, v)
            heapq.heappush(pq, (v, i, j+1))

Analysis:

In the second approach this statement make me confused.

PriorityQueue < Integer > min_queue = new PriorityQueue < Integer > ((i, j) -> nums[i][next[i]] - nums[j][next[j]]);

may be it can sort automatically in the PriorityQueue function.

C++ ------> priorityqueue:

Priority Queue in C++ Standard Template Library (STL)

Priority queues are a type of container adapters, specifically designed such that the first element of the queue is the greatest of all elements in the queue and elements are in non decreasing order(hence we can see that each element of the queue has a priority{fixed order}).
 
The functions associated with priority queue are:
empty() – Returns whether the queue is empty
size() – Returns the size of the queue
top() – Returns a reference to the top most element of the queue
push(g) – Adds the element ‘g’ at the end of the queue
pop() – Deletes the first element of the queue

#include <iostream> 
#include <queue> 
  
using namespace std; 
  
void showpq(priority_queue <int> gq) 
{ 
    priority_queue <int> g = gq; 
    while (!g.empty()) 
    { 
        cout << '\t' << g.top(); 
        g.pop(); 
    } 
    cout << '\n'; 
} 
  
int main () 
{ 
    priority_queue <int> gquiz; 
    gquiz.push(10); 
    gquiz.push(30); 
    gquiz.push(20); 
    gquiz.push(5); 
    gquiz.push(1); 
  
    cout << "The priority queue gquiz is : "; 
    showpq(gquiz); 
  
    cout << "\ngquiz.size() : " << gquiz.size(); 
    cout << "\ngquiz.top() : " << gquiz.top(); 
  
  
    cout << "\ngquiz.pop() : "; 
    gquiz.pop(); 
    showpq(gquiz); 
  
    return 0; 
} 

come from : https://www.geeksforgeeks.org/priority-queue-in-cpp-stl/