1323. Maximum 69 Number
常规解法，略显冗余

class Solution {
public:
    int maximum69Number (int num) {
        stack<int> stack;
        int nums = 0,changed = 0;
        while(num)
        {
            stack.push(num % 10);
            num /= 10;
        }
        
        while(!stack.empty())
        {
            if(stack.top() == 6 && !changed && (changed = 1))
                nums = nums * 10 + 9;
            else
                nums = nums * 10 + stack.top();
            
            stack.pop();
        }
        
        return nums;
    }
};

运行结果：

Runtime: 0 ms, faster than 100.00% of C++ online submissions for Maximum 69 Number.
Memory Usage: 6.3 MB, less than 5.08% of C++ online submissions for Maximum 69 Number.

直接转成字符串，然后替换第一个6，再转回数字：

class Solution {
public:
    int maximum69Number (int num) {
        string s = to_string(num);
        for(auto &c:s)
        {
            if(c =='6'){c='9';break;}
        }
        return stoi(s);
    }
};