add 0378 Solution for Java

Author: EndaLin

solution/0300-0399/0378.Kth Smallest Element in a Sorted Matrix/Solution.java

@@ -0,0 +1,34 @@
+class Solution {
+    public int kthSmallest(int[][] matrix, int k) {
+        int len = matrix.length;
+        int min = matrix[0][0], max = matrix[len - 1][len - 1];
+        while (min < max) {
+            int mid = min + ((max - min) >> 1);
+            if (check(matrix, mid, k, len)) {
+                max = mid;
+            } else {
+                min = mid + 1;
+            }
+        }
+
+        return min;
+    }
+
+    private boolean check(int[][] matrix, int mid, int k, int n) {
+        // 从左下角走起
+        int i = n - 1;
+        int j = 0;
+        int num = 0;
+
+        while (i >= 0 && j < n) {
+            if (matrix[i][j] <= mid) {
+                num += i + 1;
+                j++;
+            } else {
+                i--;
+            }
+        }
+
+        return num >= k;
+    }
+}