[LintCode] Search a 2D Matrix

public boolean searchMatrix(int[][] matrix, int target) {
        // write your code here
        if(matrix == null || matrix.length == 0)
            return false;
        int i = 0;
        int j = matrix[0].length-1;
        while(i < matrix.length && j >=0) {
            if(target == matrix[i][j])
                return true;
            else if(target < matrix[i][j])
                j--;
            else
                i++;
        }
        return false;
    }