Formatted question description: https://leetcode.ca/all/1351.html
1351. Count Negative Numbers in a Sorted Matrix (Easy)
Given a m * n
matrix grid
which is sorted in non-increasing order both row-wise and column-wise.
Return the number of negative numbers in grid
.
Example 1:
Input: grid = [[4,3,2,-1],[3,2,1,-1],[1,1,-1,-2],[-1,-1,-2,-3]] Output: 8 Explanation: There are 8 negatives number in the matrix.
Example 2:
Input: grid = [[3,2],[1,0]] Output: 0
Example 3:
Input: grid = [[1,-1],[-1,-1]] Output: 3
Example 4:
Input: grid = [[-1]] Output: 1
Constraints:
m == grid.length
n == grid[i].length
1 <= m, n <= 100
-100 <= grid[i][j] <= 100
Related Topics:
Array, Binary Search
Solution 1. Brute Force
// OJ: https://leetcode.com/problems/count-negative-numbers-in-a-sorted-matrix/
// Time: O(MN)
// Space: O(1)
class Solution {
public:
int countNegatives(vector<vector<int>>& grid) {
int ans = 0;
for (auto &row : grid) {
for (int c : row) {
if (c < 0) ++ans;
}
}
return ans;
}
};
Solution 2. Binary Search
// OJ: https://leetcode.com/problems/count-negative-numbers-in-a-sorted-matrix/
// Time: O(MlogN)
// Space: O(1)
class Solution {
public:
int countNegatives(vector<vector<int>>& grid) {
int ans = 0;
for (auto &row : grid) {
ans += upper_bound(row.rbegin(), row.rend(), -1) - row.rbegin();
}
return ans;
}
};
Solution 3. Search Break Points
Traverse from upper right to lower left.
// OJ: https://leetcode.com/problems/count-negative-numbers-in-a-sorted-matrix/
// Time: O(M+N)
// Space: O(1)
class Solution {
public:
int countNegatives(vector<vector<int>>& grid) {
int M = grid.size(), N = grid[0].size(), x = 0, y = N - 1, ans = 0;
while (x < M) {
while (y >= 0 && grid[x][y] < 0) --y;
ans += N - 1 - y;
++x;
}
return ans;
}
};
Java
-
class Solution { public int countNegatives(int[][] grid) { int count = 0; int rows = grid.length, columns = grid[0].length; for (int i = rows - 1; i >= 0; i--) { if (grid[i][columns - 1] >= 0) break; for (int j = columns - 1; j >= 0; j--) { if (grid[i][j] < 0) count++; else break; } } return count; } }
-
// OJ: https://leetcode.com/problems/count-negative-numbers-in-a-sorted-matrix/ // Time: O(MN) // Space: O(1) class Solution { public: int countNegatives(vector<vector<int>>& grid) { int ans = 0; for (auto &row : grid) { for (int c : row) { if (c < 0) ++ans; } } return ans; } };
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print("Todo!")