Basic Calculator II LeetCode Solution | Easy Approach

Basic Calculator II 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 [-231, 231 - 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 * 105
• 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, 231 - 1].
• The answer is guaranteed to fit in a 32-bit integer.

Basic Calculator II Solutions

✅Time: O(n)
✅Space: O(n)

C++

class Solution {
 public:
  int calculate(string s) {
    stack<int> nums;  // stores nums
    stack<char> ops;  // stores operators

    for (int i = 0; i < s.length(); ++i) {
      const char c = s[i];
      if (isdigit(c)) {
        int num = c - '0';
        while (i + 1 < s.length() && isdigit(s[i + 1])) {
          num = num * 10 + (s[i + 1] - '0');
          ++i;
        }
        nums.push(num);
      } else if (c == '+' || c == '-' || c == '*' || c == '/') {
        while (!ops.empty() && compare(ops.top(), c))
          nums.push(calculate(pop(ops), pop(nums), pop(nums)));
        ops.push(c);
      }
    }

    while (!ops.empty())
      nums.push(calculate(pop(ops), pop(nums), pop(nums)));

    return nums.top();
  }

 private:
  int calculate(char op, int b, int a) {
    switch (op) {
      case '+':
        return a + b;
      case '-':
        return a - b;
      case '*':
        return a * b;
      case '/':
        return a / b;
    }
    throw;
  }

  // return true if priority(op1) >= priority(op2)
  bool compare(char op1, char op2) {
    return op1 == '*' || op1 == '/' || op2 == '+' || op2 == '-';
  }

  char pop(stack<char>& ops) {
    const char op = ops.top();
    ops.pop();
    return op;
  }

  int pop(stack<int>& nums) {
    const int num = nums.top();
    nums.pop();
    return num;
  }
};

Java

 
 class Solution {
  public int calculate(String s) {
    Deque<Integer> nums = new ArrayDeque<>();  // stores nums
    Deque<Character> ops = new ArrayDeque<>(); // stores operators and parentheses

    for (int i = 0; i < s.length(); ++i) {
      final char c = s.charAt(i);
      if (Character.isDigit(c)) {
        int num = c - '0';
        while (i + 1 < s.length() && Character.isDigit(s.charAt(i + 1))) {
          num = num * 10 + (s.charAt(i + 1) - '0');
          ++i;
        }
        nums.push(num);
      } else if (c == '+' || c == '-' || c == '*' || c == '/') {
        while (!ops.isEmpty() && compare(ops.peek(), c))
          nums.push(calculate(ops.pop(), nums.pop(), nums.pop()));
        ops.push(c);
      }
    }

    while (!ops.isEmpty())
      nums.push(calculate(ops.pop(), nums.pop(), nums.pop()));

    return nums.peek();
  }

  private int calculate(char op, int b, int a) {
    switch (op) {
      case '+':
        return a + b;
      case '-':
        return a - b;
      case '*':
        return a * b;
      case '/':
        return a / b;
    }
    throw new IllegalArgumentException();
  }

  // return true if priority(op1) >= priority(op2)
  private boolean compare(char op1, char op2) {
    return op1 == '*' || op1 == '/' || op2 == '+' || op2 == '-';
  }
}

Python

class Solution:
  def calculate(self, s: str) -> int:
    ans = 0
    prevNum = 0
    currNum = 0
    op = '+'

    for i, c in enumerate(s):
      if c.isdigit():
        currNum = currNum * 10 + int(c)
      if not c.isdigit() and c != ' ' or i == len(s) - 1:
        if op == '+' or op == '-':
          ans += prevNum
          prevNum = currNum if op == '+' else -currNum
        elif op == '*':
          prevNum = prevNum * currNum
        elif op == '/':
          if prevNum < 0:
            prevNum = ceil(prevNum / currNum)
          else:
            prevNum = prevNum // currNum
        op = c
        currNum = 0

    return ans + prevNum

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