Search a 2D Matrix
来自 <https://leetcode.com/problems/search-a-2d-matrix/>
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 from left to right.
- The first integer of each row is greater than the last integer of the previous row.
For example,
Consider the following matrix:
[
[1, 3, 5, 7],
[10, 11, 16, 20],
[23, 30, 34, 50]
]
Given target = 3, return true.
题目解读
写一个能够从m x n 的矩阵中找出特定值的高校算法,这个矩阵有以下特点
- 每一行的整数从左到右为有序的
- 每一行的第一个数比上一行的最后一个数大。
例如
考虑如下矩阵
[
[1, 3, 5, 7],
[10, 11, 16, 20],
[23, 30, 34, 50]
]
给定target=3, 返回true.
解析一:
最直接的方法就是从前往后,从左到右进行遍历整个数组,时间复杂度为O(mn).
解析二:
由于给定的矩阵从左到右,从上到下都是有序的,所以可以首先考虑到二分查找,首先找到target大致在哪一行,然后找出其确定位置。
解析一代码(略)
解法二代码
public class Solution {
public boolean searchMatrix(int[][] matrix, int target) {
int low = 0;
int high = matrix.length-1;
int mid = (low + high) / 2;
/**
* 二分查找target大致在哪一行
*/
while(low<=high) {
mid = (low + high) / 2;
if( matrix[mid][0] == target)
return true;
else if (matrix[mid][0] < target)
low = mid+1;
else
high = mid-1;
}
//target所在的行是low和high中小的那个
int row = (low+high) / 2;
low = 0;
high = matrix[mid].length-1;
/**
* 二分查找target所在的具体位置
*/
while(low<=high) {
mid = (low + high) / 2;
if( matrix[row][mid] == target)
return true;
else if (matrix[row][mid] < target)
low = mid+1;
else
high = mid-1;
}
return false;
}
}
解法二性能