notes/OJ notes/pages/Leetcode Search-A-2D-Matrix-II.md

# Leetcode Search-A-2D-Matrix-II

2022-09-03 14:57

> ##### Algorithms:
>
> #algorithm #divide_and_conquer
>
> ##### Data structures:
>
> #DS #array
>
> ##### Difficulty:
>
> #coding_problem #difficulty-medium
>
> ##### Additional tags:
>
> #leetcode #CS_list_need_practicing
>
> ##### Revisions:
>
> N/A

##### Links:

- [Link to problem](https://leetcode.com/problems/search-a-2d-matrix-ii/)

---

### Problem

Write an efficient algorithm that searches for a value `target` in an `m x n` integer matrix `matrix`. This matrix has the following properties:

- Integers in each row are sorted in ascending from left to right.
- Integers in each column are sorted in ascending from top to bottom.

#### Examples

**Example 1:**

![](https://assets.leetcode.com/uploads/2020/11/24/searchgrid2.jpg)

```
**Input:** matrix = [[1,4,7,11,15],[2,5,8,12,19],[3,6,9,16,22],[10,13,14,17,24],[18,21,23,26,30]], target = 5
**Output:** true
```

**Example 2:**

![](https://assets.leetcode.com/uploads/2020/11/24/searchgrid.jpg)

```
**Input:** matrix = [[1,4,7,11,15],[2,5,8,12,19],[3,6,9,16,22],[10,13,14,17,24],[18,21,23,26,30]], target = 20
**Output:** false
```

#### Constraints

-   `m == matrix.length`
-   `n == matrix[i].length`
-   `1 <= n, m <= 300`
-   `-109 <= matrix[i][j] <= 109`
-   All the integers in each row are **sorted** in ascending order.
-   All the integers in each column are **sorted** in ascending order.
-   `-109 <= target <= 109`

### Thoughts

> [!summary]
> This is a #divide_and_conquer problem.

It's divide and conquer, because every time we do a action,
the problem is smaller.

Start from the top-right, (alternatively, bottom-left),
because walking left makes the number smaller, and down makes the number bigger.

### Solution

```cpp
class Solution {
public:
  bool searchMatrix(vector<vector<int>> &matrix, int target) {
    // search from top-right
    int c = matrix[0].size() - 1;
    int r = 0;
    int m = matrix.size();
    while (c >= 0 && r < m) {
      if (matrix[r][c] > target) {
        c--;
      } else if (matrix[r][c] < target) {
        r++;
      } else {
        return true;
      }
    }

    return false;
  }
};
```