/// <summary>
/// Check is parentheses are balanced
///
/// ((())) is balanced
/// )( is not balanced
/// </summary>
/// <param name="parantheses"></param>
/// <returns></returns>
public static bool BalancedParentheses(string parantheses)
{
Stack<char> tracker = new Stack<char>();
foreach (char character in parantheses)
{
if (character == '(')
{
tracker.Push(character);
}
else
{
if (tracker.Count == 0)
{
return false;
}
char existingCharacter = tracker.Pop();
if (existingCharacter != '(')
{
return false;
}
}
}
return true;
}
/// <summary>
/// Check is parentheses are balanced
///
/// Support (, {, [
/// </summary>
/// <param name="parantheses"></param>
/// <returns></returns>
public static bool BalancedParenthesesMultiple(string parantheses)
{
Dictionary<char, char> bracketMatch = new Dictionary<char, char> {
{')', '('},
{']', '['},
{'}', '{'},
};
Stack<char> tracker = new Stack<char>();
foreach (char character in parantheses)
{
if ((character == '(') || (character == '[') || (character == '{'))
{
tracker.Push(character);
}
else
{
if (tracker.Count == 0)
{
return false;
}
char existingCharacter = tracker.Pop();
if (existingCharacter != bracketMatch[character])
{
return false;
}
}
}
return true;
}
/// <summary>
/// Given a histogram, find the area of largest rectangle that can be formed
///
/// -----
/// ----| |
/// | | |
/// | | |----
/// | | | |
/// | | | |
///
/// Using a stack, push any histogram of greater height. Once we encounter a short
/// histogram, we will pop off any greater height ones and calculate the area
/// If this area is greater than the max area, then replace max
/// </summary>
/// <param name="heights"></param>
/// <returns></returns>
public static long GetLargestRectangle(int[] heights)
{
long maxArea = 0;
Stack<LargestStorage> storage = new Stack<LargestStorage>();
int index = 0;
foreach (int height in heights)
{
while ((storage.Count > 0) && (storage.Peek().Height > height))
{
LargestStorage last = storage.Pop();
long area = (index - last.Index) * last.Height;
if (area > maxArea)
{
maxArea = area;
}
}
//need to put the current one for the future
storage.Push(new LargestStorage { Index = index, Height = height });
index++;
}
//need to pop stuff off the stack
while (storage.Count > 0)
{
LargestStorage last = storage.Pop();
long area = (index - last.Index) * last.Height;
if (area > maxArea)
{
maxArea = area;
}
}
return maxArea;
}
/// <summary>
/// Valid operators are +, -, *, /. Each operand may be an integer or another expression.
///
/// ["2", "1", "+", "3", "*"] -> ((2 + 1) * 3) -> 9
/// ["4", "13", "5", "/", "+"] -> (4 + (13 / 5)) -> 6
/// </summary>
/// <param name="tokens"></param>
/// <returns></returns>
public static int EvalRPN(string[] tokens)
{
if (tokens == null)
{
throw new ArgumentException();
}
if (tokens.Length == 0)
{
return 0;
}
//looks like we only need to look back 2 when we encounter an operator
//what if we push into stack, pop 2 if we encounter an operator, evaluate it
//and push it back until we encounter another symbol
//if we are done, just pop the final result from the stack
//in case of an overflow
Stack<long> stack = new Stack<long>();
foreach (string token in tokens)
{
if ((token == "+") || (token == "-") || (token == "*") || (token == "/"))
{
if (stack.Count < 2)
{
throw new ArgumentException();
}
long right = stack.Pop();
long left = stack.Pop();
long result = 0;
switch (token)
{
case "+":
result = left + right;
break;
case "-":
result = left - right;
break;
case "*":
result = left * right;
break;
case "/":
result = left / right;
break;
}
stack.Push(result);
}
else
{
stack.Push(long.Parse(token));
}
}
if (stack.Count == 1)
{
return (int)stack.Pop();
}
throw new ArgumentException();
}
/// <summary>
/// Use a variable s to denote the start of the valid pattern
///
/// when we encounter ) at index i, there are 3 cases:
/// 1. stack is empty - no valid pattern, s = i + 1
/// 2. stack has 1 element - we found the end of a pattern...calculate length as i - s + 1...and compare to max
/// 3. stack has multiple elements - since all patterns from the top of the stack (only invalid is many ')'), meaning stack would be empty,
/// length is i - s.top...only if we push index into s
/// </summary>
/// <param name="parantheses"></param>
/// <returns></returns>
public static int LengthOfLongestBalancedParanthese(string parantheses)
{
int max = 0;
int start = 0;
Stack<int> tracker = new Stack<int>();
for (int index = 0; index < parantheses.Length; index++)
{
char chr = parantheses[index];
if (chr == '(')
{
tracker.Push(index);
}
else
{
if (tracker.Count == 0)
{
//invalid pattern...case 1
start = index + 1;
}
else
{
tracker.Pop();
if (tracker.Count == 0)
{
//case 2
max = System.Math.Max(max, index - start + 1);
}
else
{
//sub pattern...if everything afterward is invalid or we run out of closing brackets, this could be longest
//case 3
//(((()())
// _)
// ___)
// _____)
max = System.Math.Max(max, index - tracker.Peek());
}
}
}
}
return max;
}