题解 | #表达式求值#

此题没有了中括号和大括号，难度降低，但是负数不要忘记考虑。

public class Main{
	public static void main(String[] args){		Scanner sc = new Scanner(System.in);
		String str = sc.nextLine();
		List<String> arr = new ArrayList<>();
		StringBuffer sb= new StringBuffer();;
		for(int i=0;i<str.length();i++){
			char ch = str.charAt(i);
			if(Character.isDigit(ch)){
				sb.append(ch);
				if(i==str.length()-1 || i<str.length()-1 && !Character.isDigit(str.charAt(i+1))){
					arr.add(sb.toString());
					sb.setLength(0);
				}
			}
			else if(ch=='-'&&(i==0 || str.charAt(i-1)=='(')){
				sb.append(ch);
			}
			else {
				sb.append(ch);
				arr.add(sb.toString());
				sb.setLength(0);
			}
		}
		List<String> ans = truns(arr);
//		for(int i=0;i<ans.size();i++){
//			System.out.println(ans.get(i));
//		}
		int res = cal(ans);
		System.out.println(res);
	}
	public static boolean isD(String str){
		for(int i=0;i<str.length();i++){
			char ch = str.charAt(i);
			if(i==0 && ch=='-'&& str.length()>1){
				continue;
			}
			if(!Character.isDigit(ch)){
				return false;
			}
		}
		return true;
	}
	public static int getLv(String str){
		if("+".equals(str)||"-".equals(str)){
			return 1;
		}
		else if("*".equals(str)||"/".equals(str)){
			return 2;
		}
		return -1;
	}
	public static List<String> truns(List<String> arr){
		List<String> ans = new ArrayList<>();
		List<String> stack = new ArrayList<>();
		int top = -1;
		for(int i=0;i<arr.size();i++){
			String str = arr.get(i);
			if(isD(str)){
				ans.add(str);
			}
			else{
				if(top==-1||"(".equals(str)){
					stack.add(str);
					top++;
				}
				else if(getLv(str)>getLv(stack.get(top))){
					stack.add(str);
					top++;
				}
				else if(")".equals(str)){
					while(!"(".equals(stack.get(top))){
						ans.add(stack.get(top));
						stack.remove(top);
						top--;
					}
					stack.remove(top);
					top--;
				}
				else{
					while(top!=-1 && getLv(str)<=getLv(stack.get(top))){
						ans.add(stack.get(top));
						stack.remove(top);
						top--;
					}
					stack.add(str);
					top++;
				}
			}
		}
		while(top != -1){
			ans.add(stack.get(top));
			top--;
		}
		return ans;
	}
	public static int cal(List<String> ans){
		int res = 0;
		List<Integer> stack = new ArrayList<>();
		int top = -1;
		for(int i=0;i<ans.size();i++){
			if(isD(ans.get(i))){
				stack.add(Integer.parseInt(ans.get(i)));
				top++;
			}
			else if("+".equals(ans.get(i))){
				res = stack.get(top-1) + stack.get(top);
				stack.remove(top);
				top--;
				stack.set(top, res);
			}
			else if("-".equals(ans.get(i))){
				res = stack.get(top-1) - stack.get(top);
				stack.remove(top);
				top--;
				stack.set(top, res);
			}
			else if("*".equals(ans.get(i))){
				res = stack.get(top-1) * stack.get(top);
				stack.remove(top);
				top--;
				stack.set(top, res);
			}
			else if("/".equals(ans.get(i))){
				res = stack.get(top-1) / stack.get(top);
				stack.remove(top);
				top--;
				stack.set(top, res);
			}
		}
		return res;
	}
}