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.

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//时间复杂度为O(m+n)class Solution{public:	bool searchMatrix(vector
       
        
         >& matrix, int target) {		int m = matrix.size();		int n = matrix[0].size();		int i = 0, j = n - 1;		while (i < m && j >= 0)		{			if (matrix[i][j] == target)				return true;			else if (matrix[i][j] > target)				j--;			else				i++;		}		return false;	}};