LeetCode – Kth Smallest Element in a Sorted Matrix (Java)

Given a n x n matrix where each of the rows and columns are sorted in ascending order, find the kth smallest element in the matrix.

Note that it is the kth smallest element in the sorted order, not the kth distinct element.

Example:

matrix = [
[ 1, 5, 9],
[10, 11, 13],
[12, 13, 15]
],
k = 8,

return 13.

Java Solution

This problem is similar to Search a 2D Matrix II. The start point of such a sorted matrix is left-bottom corner.

public int kthSmallest(int[][] matrix, int k) {
    int m=matrix.length;
 
    int lower = matrix[0][0];
    int upper = matrix[m-1][m-1];
 
    while(lower<upper){
        int mid = lower + ((upper-lower)>>1);
        int count = count(matrix, mid);
        if(count<k){
            lower=mid+1;
        }else{
            upper=mid;
        }
    }
 
    return upper;
}
 
private int count(int[][] matrix, int target){
    int m=matrix.length;
    int i=m-1;
    int j=0;
    int count = 0;
 
    while(i>=0&&j<m){
        if(matrix[i][j]<=target){
            count += i+1;
            j++;
        }else{
            i--;
        }
    }
 
    return count;
}
Category >> Algorithms >> Others  
If you want someone to read your code, please put the code inside <pre><code> and </code></pre> tags. For example:
<pre><code> 
String foo = "bar";
</code></pre>
  • yukungao

    Sorry.. But how do you guarantee the mid(calculated by lower + (upper – lower)/2) do exist in the matrix.. What if it doesn’t?