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41 lines
813 B
41 lines
813 B
#include <bits/stdc++.h>
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using namespace std;
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const int N = 1e5 + 10;
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int n, a[N];
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//下面是用树状数组完成前缀和的保存办法
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#define lowbit(x) (x & -x)
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int c[N];
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//点更新
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void add(int i, int z) {
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for (; i <= n; i += lowbit(i)) c[i] += z; //更新所有祖先
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}
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//前缀和
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int sum(int i) {
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int s = 0;
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for (; i; i = i - lowbit(i)) s += c[i];
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return s;
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}
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//区间和
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int sum(int i, int j) {
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return sum(j) - sum(i - 1);
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}
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/*
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测试用例：
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5
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2 3 1 4 5
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*/
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int main() {
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scanf("%d", &n);
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for (int i = 1; i <= n; i++) {
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scanf("%d", &a[i]);
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add(i, a[i]);
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}
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//利用树状数组求前缀和
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printf("%d\n", sum(4));
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//利用树状数组求区间和
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printf("%d\n", sum(5) - sum(3));
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return 0;
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} |