# 【题目】Range Sums

AtCoder Beginner Contest 238

E - Range Sums

Time Limit: 2 sec / Memory Limit: 1024 MB

Score : $500$ points

### Problem Statement

Takahashi has a secret integer sequence $a$. You know that the length of $a$ is $N$.

You want to guess the contents of $a$. He has promised to give you the following $Q$ additional pieces of information.

• The $i$-th information: the value $a_{l_i}+a_{l_i+1}+\cdots+a_{r_i}$.

Is it possible to determine the sum of all elements in $a$, $a_1+a_2+\cdots+a_N$, if the $Q$ pieces of promised information are given?

### Constraints

• $1 \leq N \leq 2 \times 10^5$
• $1 \leq Q \leq \min(2 \times 10^5,\frac{N(N+1)}{2})$
• $1 \leq l_i \leq r_i \leq N$
• $(l_i,r_i) \neq (l_j,r_j)\ (i \neq j)$
• All values in input are integers.

### Input

Input is given from Standard Input in the following format:

$N$ $Q$
$l_1$ $r_1$
$l_2$ $r_2$
$\hspace{0.4cm}\vdots$
$l_Q$ $r_Q$

### Output

If it is possible to determine the sum of all elements in $a$, print Yes; otherwise, print No.

3 3
1 2
2 3
2 2

### Sample Output 1

Yes

From the first and second information, we can find the value $a_1+a_2+a_2+a_3$. By subtracting the value of $a_2$ from it, we can determine the value $a_1+a_2+a_3$.

4 3
1 3
1 2
2 3

### Sample Output 2

No

We can determine the sum of the first $3$ elements of $a$, but not the sum of all elements.

4 4
1 1
2 2
3 3
1 4

### Sample Output 3

Yes

The fourth information directly gives us the sum of all elements.

### 源码

#include <bits/stdc++.h>
#define MAXN 200010

using namespace std;

int fa[MAXN];
inline void init(int n)
{
for (int i = 0; i < n; i++)
fa[i] = i;
}

int find(int x)
{
return x == fa[x] ? x : (fa[x] = find(fa[x]));
}

inline void merge(int i, int j)
{
fa[find(i)] = find(j);
}

int main(void)
{
ios::sync_with_stdio(false);
int N, Q;
cin >> N >> Q;
init(N + 1);
while (Q--)
{
int l, r;
cin >> l >> r;
merge(l - 1, r);
}
if (find(0) == find(N))
{
cout << "Yes" << endl;
}
else
{
cout << "No" << endl;
}
return 0;
}