Basic Calculator - Stack（表达式计算器）

public class Solution {
    /**
     * @param s: the given expression
     * @return: the result of expression
     */
    public int calculate(String s) {
        // Write your code here
        Stack<Integer> stack = new Stack<Integer>();
        int result =0;
        int number =0;
        int sign =1;
        
        for(int i =0;i<s.length();i++){
            char c = s.charAt(i);
            if(Character.isDigit(c)){
                number = 10*number + (int)(c-‘0‘);
            }else if(c==‘+‘){
                result +=sign*number;
                number =0;
                sign =1;
            }else if(c == ‘-‘){
                result += sign * number;
                number = 0;
                sign = -1;
            }else if(c == ‘(‘){
                stack.push(result);
                stack.push(sign);
                sign = 1;
                result = 0;
            }else if(c == ‘)‘){
                result += sign* number;
                number =0;
                result*=stack.pop();
                result+=stack.pop();
            }
        }
        
        if(number!=0){
            result +=sign*number;
        }
        
        return result;
    }
}

public class Solution {
    /**
     * @param s: the given expression
     * @return: the result of expression
     */
    public int calculate(String s) {
        // Write your code here
        if(s==null || s.length()==0){
            return 0;
        }
        
        int len = s.length();
        Stack<Integer> stack = new Stack<Integer>();
        int num =0;
        char sign = ‘+‘;
        for(int i =0;i<len;i++){
             
            if(Character.isDigit(s.charAt(i))){
                num = num*10 + s.charAt(i)-‘0‘;
            }
        
            if((!Character.isDigit(s.charAt(i)) && ‘ ‘!=s.charAt(i)) ||i==len-1){
                if(sign ==‘-‘){
                    stack.push(-num);
                }
                if(sign == ‘+‘){
                    stack.push(num);
                }
                if(sign == ‘*‘){
                    stack.push(stack.pop()*num);
                }
                if(sign == ‘/‘){
                    stack.push(stack.pop()/num);
                }
                sign = s.charAt(i);
                num =0;
            }
        }
        
        int re =0;
        for(int i:stack){
            re +=i;
        }
        return re;
        
    }
    
}

public class Solution {
    /**
     * @param s: the expression string
     * @return: the answer
     */
    public int calculate(String s) {
        // Write your code here
        if(s==null || s.length()==0){
            return 0;
        }
        
        Stack<Integer> nums = new Stack<Integer>();
        Stack<Character> opr = new Stack<Character>();
        
        int num =0;
        for(int i=0;i<s.length();i++){
            char c = s.charAt(i);
            if(c==‘ ‘){
                continue;
            }    
            if(Character.isDigit(c)){
                num = c-‘0‘;
                while(i<s.length()-1 && Character.isDigit(s.charAt(i+1))){
                    num = num*10+ (s.charAt(i+1)-‘0‘);
                    i++;
                }
                nums.push(num);
                num =0;
            }else if(c==‘(‘){
                opr.push(c);
            }else if(c==‘)‘){
                while(opr.peek()!=‘(‘){
                    nums.push(calculate(nums.pop(),nums.pop(),opr.pop()));
                }
                opr.pop();
            }else if(c==‘+‘||c==‘-‘||c==‘*‘||c==‘/‘){
                if(!opr.isEmpty()&&needCalFirst(opr.peek(),c)){
                    nums.push(calculate(nums.pop(),nums.pop(),opr.pop()));
                }
                opr.push(c);
            }
        }
        
        while(!opr.isEmpty()){
             nums.push(calculate(nums.pop(),nums.pop(),opr.pop()));
        }
        
        return nums.pop();
    }
    
    public int calculate(int num1, int num2, char op){
        switch(op){
            case ‘+‘:return num2+num1;
            case ‘-‘:return num2-num1;
            case ‘*‘:return num2*num1;
            case ‘/‘:return num2/num1;
            default:return 0;
        }
    }
    
    public boolean needCalFirst(char firstOp, char secondOp){
        if(firstOp==‘(‘|| firstOp==‘)‘){
            return false;
        }
        
        if((firstOp==‘+‘||firstOp==‘-‘)&&(secondOp==‘*‘||secondOp==‘/‘)){
            return false;
        }
        
        return true;
    }
}

public class Solution {
    /**
     * @param expression: a list of strings
     * @return: an integer
     */
    public int evaluateExpression(String[] expression) {
        // write your code here
        if(expression==null || expression.length==0){
            return 0;
        }
        
        String s = "";
        for(int i=0;i<expression.length;i++){
            s+=expression[i];
        }
        
        Stack<Integer> nums = new Stack<Integer>();
        Stack<Character> opr = new Stack<Character>();
        
        int num =0;
        for(int i=0;i<s.length();i++){
            char c = s.charAt(i);
            if(c==‘ ‘){
                continue;
            }    
            if(Character.isDigit(c)){
                num = c-‘0‘;
                while(i<s.length()-1 && Character.isDigit(s.charAt(i+1))){
                    num = num*10+ (s.charAt(i+1)-‘0‘);
                    i++;
                }
                nums.push(num);
                num =0;
            }else if(c==‘(‘){
                opr.push(c);
            }else if(c==‘)‘){
                while(opr.peek()!=‘(‘){
                    if(nums.size()>=2)
                    nums.push(calculate(nums.pop(),nums.pop(),opr.pop()));
                }
                opr.pop();
            }else if(c==‘+‘||c==‘-‘||c==‘*‘||c==‘/‘){
                if(!opr.isEmpty()&&needCalFirst(opr.peek(),c)){
                    if(nums.size()>=2)
                    nums.push(calculate(nums.pop(),nums.pop(),opr.pop()));
                }
                opr.push(c);
            }
        }
        
        while(!opr.isEmpty()){
             if(nums.size()>=2)
             nums.push(calculate(nums.pop(),nums.pop(),opr.pop()));
        }

        if(nums.size()>0){
            return nums.pop();
        }
        
        return 0;
    }
    
    public int calculate(int num1, int num2, char op){
        switch(op){
            case ‘+‘:return num2+num1;
            case ‘-‘:return num2-num1;
            case ‘*‘:return num2*num1;
            case ‘/‘:return num2/num1;
            default:return 0;
        }
    }
    
    public boolean needCalFirst(char firstOp, char secondOp){
        if(firstOp==‘(‘|| firstOp==‘)‘){
            return false;
        }
        
        if((firstOp==‘+‘||firstOp==‘-‘)&&(secondOp==‘*‘||secondOp==‘/‘)){
            return false;
        }
        
        return true;
    }
}