原子的数量

/*
 * 题目思路：
 * 1. 与上述 Java 实现思路类似，依旧使用栈来处理括号嵌套问题。
 * 2. 使用 Java Stream 来对最终结果进行排序和拼接。
 */
import java.util.*;
import java.util.stream.Collectors;
 
public class AtomCounterStream {
    public String countOfAtoms(String formula) {
        Map<String, Integer> map = new HashMap<>();
        Stack<Map<String, Integer>> stack = new Stack<>();
        int i = 0, n = formula.length();
        while (i < n) {
            if (formula.charAt(i) == '(') {
                stack.push(map);
                map = new HashMap<>();
                i++;
            } else if (formula.charAt(i) == ')') {
                int iStart = ++i;
                int multiplicity = 0;
                while (i < n && Character.isDigit(formula.charAt(i))) {
                    multiplicity = multiplicity * 10 + formula.charAt(i++) - '0';
                }
                multiplicity = multiplicity == 0 ? 1 : multiplicity;
                if (!stack.isEmpty()) {
                    Map<String, Integer> top = map;
                    map = stack.pop();
                    for (String key : top.keySet()) {
                        map.put(key, map.getOrDefault(key, 0) + top.get(key) * multiplicity);
                    }
                }
            } else {
                int iStart = i++;
                while (i < n && Character.isLowerCase(formula.charAt(i))) i++;
                String name = formula.substring(iStart, i);
                int multiplicity = 0;
                while (i < n && Character.isDigit(formula.charAt(i))) {
                    multiplicity = multiplicity * 10 + formula.charAt(i++) - '0';
                }
                multiplicity = multiplicity == 0 ? 1 : multiplicity;
                map.put(name, map.getOrDefault(name, 0) + multiplicity);
            }
        }
        return map.entrySet().stream()
            .sorted(Map.Entry.comparingByKey())
            .map(entry -> entry.getKey() + (entry.getValue() > 1 ? entry.getValue().toString() : ""))
            .collect(Collectors.joining());
    }
}