Codeforces Round #197 (Div. 2) A. Helpful Maths

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http://codeforces.com/problemset/problem/339/A

题意：
把一个求和的式子（只包含+1，+2，+3）重新排序为递增的求和式子。
比如输入3+2+1，输出1+2+3。**

代码：

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#include <bits/stdc++.h>
using namespace std;

int main() {
    string s, out = "";
    int n1 = 0, n2 = 0, n3 = 0;
    cin >> s;
    int n = s.length();
    for (int i = 0; i < n; i++) {
        if (s[i] == '1')      n1++;
        else if (s[i] == '2') n2++;
        else if (s[i] == '3') n3++;
    }
    for (int i = 0; i < n1; i++) out += "1+";
    for (int i = 0; i < n2; i++) out += "2+";
    for (int i = 0; i < n3; i++) out += "3+";
    if (out[out.size() - 1] == '+') {
        for (int i; i < out.size() - 1; i++) cout << out[i]; // 删去最后的“+”
    }
    return 0;
};