Check if a string of brackets is balanced: ()[]{}
Implement a FIFO queue using only two stacks.
Design a stack that supports push, pop, and getMin in O(1).
Evaluate a postfix (reverse Polish notation) expression.
Check if a string of parentheses () is valid.
Reverse a string using a stack.
Traverse a graph depth-first using an explicit stack.
Evaluate an arithmetic expression in Reverse Polish Notation (postfix).