题目：

题解：

class Solution {public int reversePairs(int[] nums) {Set<Long> allNumbers = new TreeSet<Long>();for (int x : nums) {allNumbers.add((long) x);allNumbers.add((long) x * 2);}// 利用哈希表进行离散化Map<Long, Integer> values = new HashMap<Long, Integer>();int idx = 0;for (long x : allNumbers) {values.put(x, idx);idx++;}int ret = 0;BIT bit = new BIT(values.size());for (int i = 0; i < nums.length; i++) {int left = values.get((long) nums[i] * 2), right = values.size() - 1;ret += bit.query(right + 1) - bit.query(left + 1);bit.update(values.get((long) nums[i]) + 1, 1);}return ret;}
}class BIT {int[] tree;int n;public BIT(int n) {this.n = n;this.tree = new int[n + 1];}public static int lowbit(int x) {return x & (-x);}public void update(int x, int d) {while (x <= n) {tree[x] += d;x += lowbit(x);}}public int query(int x) {int ans = 0;while (x != 0) {ans += tree[x];x -= lowbit(x);}return ans;}
}