fdu Problem 2072 Count

思路：對數組非降序排序，並記錄原始下標。利用二分查找，找到要查找的數後進行區間判斷，如果在查詢區間的話，count累加；不在的話，繼續查找，仍是二分。

#include <iostream>
#include <cstdio>
#include <cstring>
#include <algorithm>

using namespace std;

const int N = 100009;

struct Arr {
    int val, it;
};

Arr a[N];

int c, d, ans;

int cmp(Arr a, Arr b) {
    if (a.val == b.val)
        return a.it < b.it;
    return a.val < b.val;
}

void bisearch(int l, int r, int x) {
    if (l > r)
        return;
    int mid;
    mid = (l + r)>>1;
    if (a[mid].val > x)
        bisearch(l, mid-1, x);
    else if (a[mid].val < x)
        bisearch(mid+1, r, x);
    else {
        if(a[mid].it > d)
            bisearch(l, mid-1, x);
        else if (a[mid].it < c)
            bisearch(mid+1, r, x);
        else {
            if (a[mid].it >= c && a[mid].it <= d)
                ans++;
            bisearch(l, mid-1, x);
            bisearch(mid+1, r, x);
        }
    }
}

int main()
{
    int i, n, q, x;
    while (scanf("%d %d", &n, &q) != EOF) {
        for (i = 1; i <= n; ++i) {
            scanf("%d", &a[i].val);
            a[i].it = i;
        }
        sort(a+1, a + n + 1, cmp);
        for (i = 0; i < q; ++i) {
            scanf("%d %d %d", &c, &d, &x);
            ans = 0;
            bisearch(1, n, x);
            printf("%d\n", ans);
        }
    }
    return 0;
}