Kth Smallest Element In A Sorted Matrix Problem


Description

LeetCode Problem 378.

Given an n x n matrix where each of the rows and columns is sorted in ascending order, return the k^th smallest element in the matrix.

Note that it is the k^th smallest element in the sorted order, not the k^th distinct element.

You must find a solution with a memory complexity better than O(n^2).

Example 1:

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Input: matrix = [[1,5,9],[10,11,13],[12,13,15]], k = 8
Output: 13
Explanation: The elements in the matrix are [1,5,9,10,11,12,13,13,15], and the 8^th smallest number is 13

Example 2:

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Input: matrix = [[-5]], k = 1
Output: -5

Constraints:

  • n == matrix.length == matrix[i].length
  • 1 <= n <= 300
  • -10^9 <= matrix[i][j] <= 10^9
  • All the rows and columns of matrix are guaranteed to be sorted in non-decreasing order.
  • 1 <= k <= n^2


Sample C++ Code

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class Solution {
public:
    int kthSmallest(vector<vector<int>>& matrix, int k) {
        int n = matrix.size();
        
        int left = matrix[0][0], right = matrix[n-1][n-1];
        int mid;
        
        while (left < right) {
            mid = (left + right) / 2;
            int lessthanmid = 0;
            int i = n-1, j = 0;
            while (i >= 0 && j < n) {
                if (matrix[i][j] <= mid) {
                    j ++;
                    lessthanmid += i+1;
                } else {
                    i --;
                }
            }
            if (lessthanmid < k) {
                left = mid + 1;
            } else {
                right = mid;
            }
            
        }
        return left;
        
    }
};




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