问题描述
给定一个字符串表达式，只包含数字、空格和 +、-、*、/，已知这个表达式合法，且不会出现负数，对于除法，向下取整，即 3 / 2 = 1。要求求出这个表达式的值。题目链接：**点我**
样例输入输出
输入：5 / 2
输出：2
输入：4 + 3 *2
输出：10
问题解法
针对表达式求值，将中缀表达式转成后缀表达式，然后运用栈进行求解。针对此题，只包含加减乘除，并没有括号，相比而言会简单点。代码如下
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| class Solution
{
public int calculate(String s)
{
Stack<Character> opStack = new Stack<>();
Stack<Integer> numStack = new Stack<>();
int num = 0;
boolean isAdd = false;
for (int i = 0; i < s.length(); i++)
{
char ch = s.charAt(i);
if (Character.isDigit(ch))
{
num = num * 10 + (ch - '0');
isAdd = true;
continue;
}
if (isAdd)
{
numStack.push(num);
num = 0;
isAdd = false;
}
if (ch == ' ')
{
continue;
}
while (!opStack.isEmpty() && !isHigh(ch, opStack.peek()))
{
int second = numStack.pop();
int first = numStack.pop();
numStack.push(calc(first, second, opStack.pop()));
}
opStack.push(ch);
}
if (isAdd)
{
numStack.push(num);
}
while (!opStack.isEmpty())
{
int second = numStack.pop();
int first = numStack.pop();
numStack.push(calc(first, second, opStack.pop()));
}
return numStack.pop();
}
private int calc(int first, int second, char op)
{
if (op == '+')
{
return first + second;
}
if (op == '-')
{
return first - second;
}
if (op == '*')
{
return first * second;
}
return first / second;
}
private boolean isHigh(char first, char second)
{
return (first == '*' || first == '/') && (second == '+' || second == '-');
}
}
|