题解 | #表达式求值#

import java.util.*;


public class Solution {
    /**
     * 代码中的类名、方法名、参数名已经指定，请勿修改，直接返回方法规定的值即可
     *
     * 返回表达式的值
     * @param s string字符串 待计算的表达式
     * @return int整型
     */
    public int solve (String s) {
        Stack<Integer> stack = new Stack<>();
        for ( int i = 0; i < s.length(); i++) {
            char c = s.charAt(i);
            if (c ==  '(') {
                stack.push(i);
            }
        }
        if (stack.isEmpty()) {
            return computeResult(s);
        }
        StringBuilder builder = new StringBuilder(s);
        while (!stack.isEmpty()) {
            Integer pop = stack.pop();
            int index = pop;
            int indexOf = builder.indexOf(")", index);
            String substring = builder.substring(index + 1, indexOf);
            if (substring.length() > 0) {
                int compute = computeResult(substring);
                builder.replace(index, indexOf + 1, compute + "");
            } else {
                builder.replace(index + 1, indexOf, substring);
            }
        }
        return  computeResult(builder.toString());
    }

    public int computeResult(String s) {
        List<Object> arr = new ArrayList<>();
        compute(s, 0, arr);
        Integer res = null;
        Character reg = null;
        for (Object ite : arr) {
            if (ite instanceof Integer) {
                if (reg == null) {
                    res = (Integer) ite;
                } else {
                    if ('-' == reg) {
                        res = res - (Integer) ite;
                    } else {
                        res = res + (Integer) ite;
                    }
                }
            }
            if (ite instanceof Character) {
                reg = (Character) ite;
            }
        }
        return res;
    }

    public void compute (String s, int index, List<Object> arr) {
        int curri = index;
        StringBuilder numpar = new StringBuilder();
        List<Character> regres = new ArrayList<Character>() {
            {
                add('+');
                add('-');
                add('*');
            }
        };

        while ( curri < s.length() && (!regres.contains(s.charAt(curri))
                                       || ('-' == s.charAt(curri) && (curri == 0))
                                       || regres.contains(s.charAt(curri - 1)) )) {
            numpar.append(s.charAt(curri));
            curri++;
        }
        int i = Integer.parseInt(numpar.toString());
        if (curri >= s.length() - 1) {
            arr.add(i);
            return;
        }
        if ('*' == s.charAt(curri)) {
            while (curri < s.length() && '*' == s.charAt(curri)) {
                numpar = new StringBuilder();
                curri++;
                while ( curri < s.length() && (!regres.contains(s.charAt(curri))
                                               || ('-' == s.charAt(curri) && numpar.length() == 0))) {
                    numpar.append(s.charAt(curri));
                    curri++;
                }
                i =  i * Integer.parseInt(numpar.toString());
            }
            if (curri >= s.length() - 1) {
                arr.add(i);
                return;
            }
            if ('+' == s.charAt(curri)) {
                arr.add(i);
                arr.add('+');
                compute(s, ++curri, arr);
            } else {
                arr.add(i);
                arr.add('-');
                compute(s, ++curri, arr);
            }
        } else {
            if ('+' == s.charAt(curri)) {
                arr.add(i);
                arr.add('+');
                compute(s, ++curri, arr);
            } else {
                arr.add(i);
                arr.add('-');
                compute(s, ++curri, arr);
            }
        }

    }
}