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Commit 73f7d1a

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Merge pull request #10 from borrelunde/problem_1351_count_negative_numbers_in_a_sorted_matrix
LeetCode problem: 1351. Count Negative Numbers in a Sorted Matrix
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  • src
    • main/java/com/borrelunde/leetcodesolutions/problem1351/countnegativenumbersinasortedmatrix
    • test/java/com/borrelunde/leetcodesolutions/problem1351/countnegativenumbersinasortedmatrix

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# 1351. Count Negative Numbers in a Sorted Matrix
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Difficulty: `Easy`
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Topics: `Array`, `Binary Search`, `Matrix`
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Given a `m x n` matrix `grid` which is sorted in non-increasing order both row-wise and column-wise, return *the number
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of **negative** numbers in `grid`*.
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**Example 1:**
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```text
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Input: grid = [[4,3,2,-1],[3,2,1,-1],[1,1,-1,-2],[-1,-1,-2,-3]]
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Output: 8
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Explanation: There are 8 negatives number in the matrix.
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```
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**Example 2:**
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```text
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Input: grid = [[3,2],[1,0]]
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Output: 0
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```
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**Constraints:**
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- `m == grid.length`
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- `n == grid[i].length`
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- `1 <= m, n <= 100`
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- `-100 <= grid[i][j] <= 100`
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**Follow up:** Could you find an `O(n + m)` solution?
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package com.borrelunde.leetcodesolutions.problem1351.countnegativenumbersinasortedmatrix;
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/**
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* This is the solution to the LeetCode problem: 1351. Count Negative Numbers in
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* a Sorted Matrix
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*
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* @author Børre A. Opedal Lunde
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* @since 2024年01月30日
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*/
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public class Solution {
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public int countNegatives(int[][] grid) {
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// This is the sum of negative numbers that will be returned.
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int sum = 0;
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// Perform a binary search in every row to find the number of negative
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// numbers in each. That number is added to the sum.
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for (final int[] row : grid) {
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int low = 0;
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int high = row.length;
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while (low < high) {
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// Bitwise right shift (>> 1) divides by two to get the middle.
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final int searchIndex = low + ((high - low) >> 1);
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// The target is (-1), the first possible negative number.
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if (row[searchIndex] > - 1) {
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low = searchIndex + 1;
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} else {
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high = searchIndex;
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}
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}
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// Low represents the index of the first negative number in the row.
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// Given we know all elements after it are negative, we can use it
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// to calculate the number of negative numbers instead of actually
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// counting them.
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sum += row.length - low;
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}
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// Return the sum of negative numbers.
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return sum;
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}
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}
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package com.borrelunde.leetcodesolutions.problem1351.countnegativenumbersinasortedmatrix;
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import org.junit.jupiter.api.DisplayName;
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import org.junit.jupiter.api.Test;
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import org.junit.jupiter.params.ParameterizedTest;
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import org.junit.jupiter.params.provider.Arguments;
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import org.junit.jupiter.params.provider.MethodSource;
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import java.util.stream.Stream;
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import static org.junit.jupiter.api.Assertions.assertEquals;
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/**
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* This is the test to the LeetCode problem: 1351. Count Negative Numbers in a
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* Sorted Matrix
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*
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* @author Børre A. Opedal Lunde
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* @since 2024年01月30日
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*/
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@DisplayName("Count Negative Numbers in a Sorted Matrix")
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class SolutionTest {
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private final Solution solution = new Solution();
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@Test
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@DisplayName("Example one")
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void exampleOne() {
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int[][] grid = {
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{4, 3, 2, - 1},
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{3, 2, 1, - 1},
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{1, 1, - 1, - 2},
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{- 1, - 1, - 2, - 3}
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};
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int expected = 8;
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int actual = solution.countNegatives(grid);
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assertEquals(expected, actual);
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}
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@Test
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@DisplayName("Example two")
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void exampleTwo() {
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int[][] grid = {
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{3, 2},
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{1, 0}
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};
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int expected = 0;
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int actual = solution.countNegatives(grid);
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assertEquals(expected, actual);
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}
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@Test
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@DisplayName("Should return zero when grid is empty")
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void shouldReturnZeroWhenGridIsEmpty() {
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int[][] grid = {{}};
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int expected = 0;
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int actual = solution.countNegatives(grid);
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assertEquals(expected, actual);
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}
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@Test
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@DisplayName("Should return zero when grid is one positive element")
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void shouldReturnZeroWhenGridIsOnePositiveElement() {
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int[][] grid = {
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{1}
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};
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int expected = 0;
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int actual = solution.countNegatives(grid);
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assertEquals(expected, actual);
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}
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@Test
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@DisplayName("Should return one when grid is one negative element")
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void shouldReturnOneWhenGridIsOneNegativeElement() {
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int[][] grid = {
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{- 1}
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};
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int expected = 1;
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int actual = solution.countNegatives(grid);
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assertEquals(expected, actual);
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}
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static Stream<Arguments> provideGrids() {
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return Stream.of(
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Arguments.of(new int[][]{
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{0, 0},
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{0, 0}
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}, 0),
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Arguments.of(new int[][]{
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{2, 1},
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{2, 1}
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}, 0),
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Arguments.of(new int[][]{
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{- 1, - 1},
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{- 1, - 1}
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}, 4),
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Arguments.of(new int[][]{
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{- 1, - 2},
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{- 1, - 2}
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}, 4),
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Arguments.of(new int[][]{
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{5, 4, 3, 2, 1},
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{4, 3, 2, 1, 0},
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{3, 2, 1, 0, - 1},
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{2, 1, 0, - 1, - 2},
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{1, 0, - 1, - 2, - 3},
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}, 6)
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);
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}
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@ParameterizedTest(name = "{1} negative numbers in {0}")
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@DisplayName("Count negative numbers in various sorted matrices")
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@MethodSource("provideGrids")
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void countNegativeNumbersInVariousSortedMatrices(int[][] grid, int expected) {
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int actual = solution.countNegatives(grid);
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assertEquals(expected, actual);
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}
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}

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