The type of verse used in both, Walt Whitman's "I Hear America Singing" and in Langston Hughes's "I, Too", is the free verse.
Free verse is a literary device that 'frees' the author from metrical structures and rythm. Rhymes do not follow any prestablished rules and the 'music' of the composition is due to the use of rhytmical phrases and literary pauses. To put it into other words, the poem has not a predefined shape since it's up to the author's will to build it in the way that fits more acordingly to what they are trying to express.