When validating bracket strings, we need to ensure that every opening bracket has a matching closing bracket in the correct order. This is a perfect use case for a stack data structure.
A stack follows the "Last In, First Out" (LIFO) principle - imagine a stack of plates where you can only add or remove plates from the top. In programming, we use lists in Python as stacks, where we append() items to the end and pop() items from the end.
The key insight for bracket validation is that when we encounter a closing bracket, it must match the most recently opened bracket that hasn't been closed yet. This "most recent" behavior is exactly what a stack gives us!
Here's how the algorithm works:
The stack helps us keep track of the "nesting" structure. Each time we open a bracket, we're going one level deeper. Each time we close a bracket, we're coming back up one level. If we try to close a bracket when there's nothing to close, or if we close the wrong type, we know the string is invalid.
1# Using a stack to reverse a string
2def reverse_string(text):
3 stack = []
4
5 # Push all characters onto the stack
6 for char in text:
7 stack.append(char) # Add to top of stack
8
9 result = ""
10 # Pop characters off the stack
11 while stack:
12 result += stack.pop() # Remove from top of stack
13
14 return result
15
16print(reverse_string("hello")) # Output: "olleh"