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Update README.md of stack (#2671)

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Richa Srivastava
2021-10-24 12:48:49 +05:30
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DataStructures/Stacks/README.md
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# STACK # STACK
stack is an ADT (abstract data type ) that act like list of objects but there is a diffrents. Stack is an ADT (abstract data type) that acts like a list of objects but there is a difference.
stack act is _LIFO_ (Last In First Out), it means that when we want to get an element from the stack we get the last element in the stack. Stack works on the principle of _LIFO_ (Last In First Out), it means that the last item added to the stack will be the first item to be removed.
stack is based on two methods (functions) Stack is based on two methods (functions)-
## push(element) ## push(element)
add "element" to the top of the stack. It adds "element" to the top of the stack.
for example: we have `1, 3, 5` in stack, then we call push(9), For example: If we have `1, 3, 5` in stack, and we call push(9),
`9` will add to last index of stack -> `1, 3, 5 , 9` `9` will be added to last index of stack -> `1, 3, 5 , 9`.
## peek() or top() ## peek() or top()
return element at the top of the stack. It returns element at the top of the stack.
for example: we have `1, 3, 5` in stack, then we call peek(), For example: If we have `1, 3, 5` in stack, and we call peek(),
`5` will be returned (without removing it from the stack) `5` will be returned (without removing it from the stack).
## pop() ## pop()
remove the last element (i.e. top of stack) from stack. It removes the last element (i.e. top of stack) from stack.
for example: we have `1, 3, 5 , 9` in stack, then we call pop(),
For example: If we have `1, 3, 5 , 9` in stack, and we call pop(),
the function will return `9` and the stack will change to `1, 3, 5`. the function will return `9` and the stack will change to `1, 3, 5`.