LeetCode227.BasicCalculatorII（基本计算器II）

原题网址：https://leetcode.com/problems/basic-calculator-ii/

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

思路：和上一道Basic Calculator题目相似，使用栈解决运算优先级的问题。

public class Solution {
    public int calculate(String s) {
        char[] sa = s.toCharArray();
        Expression[] expressiOns= new Expression[4];
        int[] operands = new int[4];
        int size = 0;
        for(int i=0; i= '0' && sa[i] <= '9') {
                // 遇到数字
                int num = sa[i] - '0';
                // 完整读取数字
                while (i= '0' && sa[i+1] <= '9') num = num*10 + (sa[++i]-'0');
                if (size > 0 && expressions[size-1] == Expression.MULTIPLY) {
                    // 如果当前的运算符为乘法，则立即完成运算
                    operands[size-2] *= num;
                    size --;
                } else if (size > 0 && expressions[size-1] == Expression.DIVIDE) {
                    // 如果当前的运算符为除法，则立即完成运算
                    operands[size-2] /= num;
                    size --;
                } else {
                    // 当前运算符为加法或减法，要等到下一个运算符才能决定
                    expressions[size] = Expression.OPERAND;
                    operands[size] = num;
                    size ++;
                }
            } else if (sa[i] == '+' || sa[i] == '-') {
                if (size > 2) {
                    // 当前运算符为加法、减法，可以检查前面是否有为完成运算的加减法
                    if (expressions[size-2] == Expression.PLUS) {
                        operands[size-3] += operands[size-1];
                        size -= 2;
                    } else if (expressions[size-2] == Expression.MINUS) {
                        operands[size-3] -= operands[size-1];
                        size -= 2;
                    }
                }
                // 加减号入栈
                if (sa[i] == '+') {
                    expressions[size++] = Expression.PLUS;
                } else {
                    expressions[size++] = Expression.MINUS;
                }
            } else {
                // sa[i] == '*' || sa[i] == '/'
                // 乘除号入栈
                if (sa[i] == '*') expressions[size++] = Expression.MULTIPLY;
                else expressions[size++] = Expression.DIVIDE;
            }
        }
        if (size > 1) {
            if (expressions[1] == Expression.PLUS) {
                operands[0] += operands[2];
            } else {
                operands[0] -= operands[2];
            }
            size = 1;
        }
        return operands[0];
    }
}
enum Expression {OPERAND, PLUS, MINUS, MULTIPLY, DIVIDE};