LeetCode 726. Number of Atoms 原子的数量

726. Number of Atoms 原子的数量

题目描述

Given a chemical formula (given as a string), return the count of each atom.

An atomic element always starts with an uppercase character, then zero or more lowercase letters, representing the name.

1 or more digits representing the count of that element may follow if the count is greater than 1. If the count is 1, no digits will follow. For example, H2O and H2O2 are possible, but H1O2 is impossible.

Two formulas concatenated together produce another formula. For example, H2O2He3Mg4 is also a formula.

A formula placed in parentheses, and a count (optionally added) is also a formula. For example, (H2O2) and (H2O2)3 are formulas.

Given a formula, output the count of all elements as a string in the following form: the first name (in sorted order), followed by its count (if that count is more than 1), followed by the second name (in sorted order), followed by its count (if that count is more than 1), and so on.

示例:

Example 1:

Input: 
formula = "H2O"
Output: "H2O"
Explanation: 
The count of elements are {'H': 2, 'O': 1}.

Example 2:

Input: 
formula = "Mg(OH)2"
Output: "H2MgO2"
Explanation: 
The count of elements are {'H': 2, 'Mg': 1, 'O': 2}.

Example 3:

Input: 
formula = "K4(ON(SO3)2)2"
Output: "K4N2O14S4"
Explanation: 
The count of elements are {'K': 4, 'N': 2, 'O': 14, 'S': 4}.

Notes:

• All atom names consist of lowercase letters, except for the first character which is uppercase.
• he length of formula will be in the range [1, 1000].
• formula will only consist of letters, digits, and round parentheses, and is a valid formula as defined in the problem.

解答

这道题是一道典型的递归求解题。

我们用map来存结果，然后定义两个函数countAlpha和countDigit，分别用来返回分子和分子的个数。

我们通过不停的调用countAlpha和countDigit，来获取分子和分子的个数，把结果加到map中。

当遇见 “(” 时，我们递归调用自己，然后把返回的结果加到 map中去。

在这个过程中，我们维持一个全局变量i，来表示我们正在处理formula的位置。

代码

class Solution {
int i;
public:
    string countOfAtoms(string formula) {
    	string ans;
    	map<string, int> res = countAtoms(formula);
    	for (auto &s : res) {
    		ans += s.first;
    		if (s.second > 1 ) 
    			ans += to_string(s.second);
    	}
    	return ans;

    }
    map<string, int> countAtoms(string formula) {
    	map<string, int> count;
    	while (i < formula.size()) {
    		if (formula[i] == '(') {
    			i++;

    			auto insideres = countAtoms(formula);
    			auto digits = countDigit(formula);
    			for (auto &s : insideres) {
    				count[s.first] += s.second * digits;
    			}


    		}
    		else if (formula[i] == ')') {

    			return count;
    		}

    			string str = countAlpha(formula);
    			auto digits = countDigit(formula);
    			count[str] += digits;
    		}
    	}
    	return count;
    }
    int countDigit(string formula) {
    	string digits = "";
    	while (isdigit(formula[i]) && i < formula.size()) {
    		digits += formula[i];
    		i ++;
    	}
    	return digits == "" ? 1 : stoi(digits);
    }
    string countAlpha(string formula) {
    	string atom = "";
    	while (i < formula.size() && (atom == "" || islower(formula[i]))) {
    		atom += formula[i];
    		i++;
    	}
    	return atom;
    }

};