POJ1788, Building a New Depot

链接：http://poj.org/problem?id=1788

“The posts are only at points where the fence changes the direction”，也就是说每个post都是拐角处。
先按X为第一关键字 Y为第二关键字排序 2n，2n-1两个相邻的y之差就是坚着的栅栏；横栅栏同理。。遍历一下加起来就ok了。
wa两次，第一次在计算长度时i每次增加1而不是增加2，第二次忘记初始化vector。。。还是不熟悉stl用法啊
Memory: 260K      Time: 16MS

#include <iostream>
#include <vector>
#include <algorithm>

using namespace std;

typedef struct 
{
    int x, y;
}point;

bool cmpX(point elem1, point elem2)
{
    if (elem1.x == elem2.x)
        return elem1.y < elem2.y;
    return elem1.x < elem2.x;
}

bool cmpY(point elem1, point elem2)
{
    if (elem1.y == elem2.y)
        return elem1.x < elem2.x;
    return elem1.y < elem2.y;
}

int main(void)
{
    int num;
    int len;
    point tmp;
    vector<point> post;

    while (cin >> num && num)
    {
        len = 0;
        post.clear();
        for (int i = 0; i < num; ++i)
        {
            cin >> tmp.x >> tmp.y;
            post.push_back(tmp);
        }
        sort(post.begin(), post.end(), cmpX);
        for (int i = 0; i < post.size() - 1; i += 2)
        {
            if (post[i].x == post[i + 1].x)
                len += (post[i + 1].y - post[i].y);
        }
        sort(post.begin(), post.end(), cmpY);
        for (int i = 0; i < post.size() - 1; i += 2)
        {
            if (post[i].y == post[i + 1].y)
                len += (post[i + 1].x - post[i].x);
        }
        cout << "The length of the fence will be " << len << " units." << endl;
    }
    return 0;
}