[LeetCode] 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.

For 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

.

解题思路

O(m+n)。从矩阵右上角开始遍历元素，若小于目标，则说明第该行元素都不符合要求，行号加1；若大于目标，则说明该列元素都不符合要求，列号减1。

实现代码

// Runtime: 14 ms
public class Solution {
public boolean searchMatrix(int[][] matrix, int target) {
if (matrix.length == 0) {
return false;
}
int i = 0;
int j = matrix[0].length - 1;

while (i < matrix.length && j >= 0) {
if (matrix[i][j] > target) {
j--;
} else if (matrix[i][j] < target) {
i++;
} else {
return true;
}
}

return false;
}
}