Kth Smallest Element in a Sorted Matrix-LeetCode#378

378. Kth Smallest Element in a Sorted Matrix

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.

Note: 
You may assume k is always valid, 1 ≤ k ≤ n².
矩阵中第K小的元素

它是排序顺序中的第k个最小元素，而不是第k个不同元素。
思路：循环遍历每个元素后，添加到一个 List ，使用 Collections.sort() 方法，排序后取 K – 1 位

public int kthSmallest(int[][] matrix, int k) {
    if (matrix.length <= 0 || matrix[0].length <= 0) return 0;
    List<Integer> tempList = new ArrayList<>();
    for (int i = 0; i < matrix.length; i++){
        for (int j = 0; j < matrix[0].length; j++) {
            tempList.add(matrix[i][j]);
        }
    }
    Collections.sort(tempList);
    return tempList.get(k - 1);
}