leetcode:Basic Calculator II

最新推荐文章于 2024-08-24 03:50:28 发布

martin_liang

最新推荐文章于 2024-08-24 03:50:28 发布

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分类专栏： C++/C leetcode 算法

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本文介绍了一个简易计算器的设计与实现，该计算器能解析并计算简单的数学表达式，包括加、减、乘、除运算，并且考虑了整数除法的取整问题。通过使用栈和向量等数据结构来处理表达式的读取和计算过程。

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Implement a basic calculator to evaluate a simple expression string.

The expression string contains only non-negative integers, +, -, *, / operators and empty spaces . The integer division should truncate toward zero.

You may assume that the given expression is always valid.

Some examples:

"3+2*2" = 7
" 3/2 " = 1
" 3+5 / 2 " = 5

Note: Do not use the eval built-in library function.

class Solution {
public:

    int getNextNum(vector<char> &str, int &idx) {
        
        int leftPos, rightPos;
        leftPos = rightPos = idx;
        
        while (rightPos < str.size() && (str[rightPos] >='0' && str[rightPos] <= '9')) {
            rightPos++;
        }
        
        int ret = 0;
        for (int i=leftPos; i<rightPos; i++) {
            ret = ret*10+(str[i]-'0');
        }
        
        idx = rightPos;
        return ret;
    }

    int calculate(string s) {
        vector<char> str;
        
        for (int i=0; i<s.size(); i++)
            if (s[i] != ' ')
                str.push_back(s[i]);
        
        int i=0;
        stack<int> tmpResult;
        int preNum, nextNum;
        
        preNum = getNextNum(str, i);
        tmpResult.push(preNum);
        
        while (i<str.size()) {
            
            int curIdx = i;
            nextNum = getNextNum(str, ++i);
            
            if (str[curIdx] == '*') {
                tmpResult.pop();
                tmpResult.push(preNum*nextNum);
            }
            else if (str[curIdx] == '/') {
                tmpResult.pop();
                tmpResult.push(preNum/nextNum);
            }
            else if (str[curIdx] == '-') {
                tmpResult.push(-nextNum);
            }
            else {
                tmpResult.push(nextNum);
            }
            
            preNum = tmpResult.top();
        }
        
        int retValue = tmpResult.top();
        tmpResult.pop();
        
        while (tmpResult.size() > 0) {
            retValue += tmpResult.top();
            tmpResult.pop();
        }
        
        return retValue;
    }
};