add: 有序矩阵中第K小的元素

README.md

@@ -381,6 +381,11 @@ LeetCode 与 LintCode 解题记录。此为个人练习仓库，代码中对重
   - LeetCode 718. 最长重复子数组 <https://leetcode-cn.com/problems/maximum-length-of-repeated-subarray/>
   - LintCode 79. 最长公共子串 <https://www.lintcode.com/problem/longest-common-substring/description>
 
+- [有序矩阵中第K小的元素](src/array/kth-smallest-element-in-a-sorted-matrix.js)
+
+  - LeetCode 378. 有序矩阵中第K小的元素 <https://leetcode-cn.com/problems/kth-smallest-element-in-a-sorted-matrix/>
+  - LintCode 401. 排序矩阵中的从小到大第k个数 <https://www.lintcode.com/problem/kth-smallest-number-in-sorted-matrix/description>
+
 ## 栈
 
 - [最大矩阵](src/stack/maximal-rectangle.js)

src/array/kth-smallest-element-in-a-sorted-matrix.js

@@ -0,0 +1,8 @@
+/**
+ * @param {number[][]} matrix
+ * @param {number} k
+ * @return {number}
+ */
+export const kthSmallest = function (matrix, k) {
+  return matrix.flat().sort((a, b) => a - b)[k - 1]
+}

test/array/kth-smallest-element-in-a-sorted-matrix.test.js

@@ -0,0 +1,20 @@
+import { kthSmallest } from '../../src/array/kth-smallest-element-in-a-sorted-matrix.js'
+
+test('有序矩阵中第K小的元素', () => {
+  expect(kthSmallest([
+    [1, 5, 9],
+    [10, 11, 13],
+    [12, 13, 15]
+  ], 8)).toBe(13)
+
+  expect(kthSmallest([
+    [1, 5, 7],
+    [3, 7, 8],
+    [4, 8, 9]
+  ], 4)).toBe(5)
+
+  expect(kthSmallest([
+    [1, 2],
+    [3, 4]
+  ], 3)).toBe(3)
+})