240. Search a 2D Matrix II (Medium)
https://leetcode.com/problems/search-a-2d-matrix-ii/
Write an efficient algorithm that searches for a value in an m x n 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.
Example:
Consider the following 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] ]
Given target = 5
, return true
.
Given target = 20
, return false
.
Solutions
class Solution {
// It is not recommended to adopt naive binary search since the time complexity will be n*log(n) whether you search
// horizontally or vertically.
// If we stand on the top-right corner of the matrix and look diagonally, it's kind of like a
// BST, we can go through this matrix to find the target like how we traverse the BST. The computation time
// scale will be decreased to O(n + m)
public boolean searchMatrix(int[][] matrix, int target) {
if (matrix == null || matrix.length == 0 || matrix[0].length == 0)
return false;
int n = matrix.length, m = matrix[0].length;
int i = 0, j = m - 1;
while (i < n && j >= 0) {
if (matrix[i][j] == target) {
return true;
}
if (matrix[i][j] > target) {
j--;
} else {
i++;
}
}
return false;
}
}