LeetCode 74. 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.
Example 1:Input:
matrix = [
[1, 3, 5, 7],
[10, 11, 16, 20],
[23, 30, 34, 50]
]
target = 3
Output: true
Example 2:Input:
matrix = [
[1, 3, 5, 7],
[10, 11, 16, 20],
[23, 30, 34, 50]
]
target = 13
Output: false

class Solution {
public:
    bool searchMatrix(vector<vector<int>>& matrix, int target) {
        int i=0;
        int m = matrix.size();
        if (!m) return false;
        int n = matrix[0].size();
        if (!n) return false;
        if(target<matrix[0][0]||target>matrix[m-1][n-1])
            return false;
        for(i=0;i<m;i++){
            if(target>=matrix[i][0]&&target<=matrix[i][n-1])
                break;
        }
        if(i==m)  //注意，如果夹在矩阵两行之间，那么遍历之后会越界，这里需要判断一下
            return false;
        for(int j=0;j<n;j++)
            if(target==matrix[i][j])
                return true;
        return false;
    }
};