LeetCode_74 Search a 2D Matrix

题目：

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

.
分析：
class Solution {
public boolean searchMatrix(int[][] matrix, int target) {
//给定一个二维矩阵，其特征为：每行数据递增排序，每一行的第一个数据大于上一行的最后一个数据
//思路：从最下面一排的第一个开始选择比较,依次往上走

if(matrix==null||matrix.length==0||matrix[0].length==0){
return false;
}
int i=matrix.length-1;
int j=0;
while(i>=0&&j<=matrix[0].length-1){
if(matrix[i][j]==target){
return true;
}else if(matrix[i][j]>target){
i--;
}else{
j++;
}
}
return false;
}
}