int a = 42; // int to string
string res = std::to_string(42); // it is a C++11 feature

stringstream ss; // int, char etc. to string
ss << a;
string res2 = ss.str();

char buffer[MAX]; // int to string
itoa(a, buffer); // itoa is not a C/C++ standard function
string res3 = string(buffer);

sprintf(buffer, "%d is %d", a, a); //int to string
string res4 = string(buffer);

int res5 = atoi(buffer); // string to int
// similar function: atol, atof, strtol

string str = "hello";
char* res5 = str.c_str(); // string to char*

char c = '1'; // one char to string
string res6 = string(1, c);

vector<int> a; // a = {2, 4, 3, 6, 5};
std::sort(a.begin(), a.end());

/**
 * Definition for an interval.
 * struct Interval {
 *     int start;
 *     int end;
 *     Interval() : start(0), end(0) {}
 * };
 */
struct customSort {
    inline bool operator()(Interval& a, Interval& b){
        return a.start < b.start;
    }
};
vector<Interval>& intervals;
std::sort(intervals.begin(), intervals.end(), customSort());