Basic Calculator II

LeetCode 227 | Difficulty: Medium

Medium

Problem Description

Given a string s which represents an expression, evaluate this expression and return its value.

The integer division should truncate toward zero.

You may assume that the given expression is always valid. All intermediate results will be in the range of [-2^31, 2^31 - 1].

Note: You are not allowed to use any built-in function which evaluates strings as mathematical expressions, such as eval().

Example 1:

Input: s = "3+2*2"
Output: 7

Example 2:

Input: s = " 3/2 "
Output: 1

Example 3:

Input: s = " 3+5 / 2 "
Output: 5

Constraints:

- `1 <= s.length <= 3 * 10^5`

- `s` consists of integers and operators `('+', '-', '*', '/')` separated by some number of spaces.

- `s` represents **a valid expression**.

- All the integers in the expression are non-negative integers in the range `[0, 2^31 - 1]`.

- The answer is **guaranteed** to fit in a **32-bit integer**.

Topics: Math, String, Stack

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Approach

Stack

Use a stack (LIFO) to track elements that need future processing. Process elements when a "trigger" condition is met (e.g., finding a smaller/larger element). Monotonic stack maintains elements in sorted order for next greater/smaller element problems.

When to use

Matching brackets, next greater element, evaluating expressions, backtracking history.

Mathematical

Look for mathematical patterns or formulas. Consider: modular arithmetic, GCD/LCM, prime factorization, combinatorics, or geometric properties.

When to use

Problems with clear mathematical structure, counting, number properties.

String Processing

Consider character frequency counts, two-pointer approaches, or building strings efficiently. For pattern matching, think about KMP or rolling hash. For palindromes, expand from center or use DP.

When to use

Anagram detection, palindrome checking, string transformation, pattern matching.

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Solutions

Solution 1: C# (Best: 121 ms)

Metric	Value
Runtime	121 ms
Memory	37.6 MB
Date	2022-01-24

Solution

public class Solution {
    public int Calculate(string s) {
       Stack<int> st = new Stack<int>();
    int current = 0; char sign = '+';
    for (int i = 0; i < s.Length; i++)
    {
        char c = s[i];
        if(char.IsDigit(c))
        {
            current = current*10 + (c-'0');
        }
        if((!char.IsDigit(c) && c != ' ') || i==s.Length-1)
        {
            if(sign == '+') st.Push(current);
            if(sign == '-') st.Push(-current);
            if(sign == '*') st.Push(st.Pop()*current);
            if(sign=='/') st.Push(st.Pop()/current);
            sign = c;
            current = 0;
        }
    }
    return st.Sum();
    }
}

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Complexity Analysis

Approach	Time	Space
Stack	$O(n)$	$O(n)$

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Interview Tips

Key Points

• Discuss the brute force approach first, then optimize. Explain your thought process.
• Think about what triggers a pop: is it finding a match, or finding a smaller/larger element?