count_of_smaller_numbers_after_self

Author: hitzzc

README.md

@@ -228,6 +228,7 @@ Golang solution for leetcode. For each problem, there is a simple *_test.go to t
 #### [309. Best Time to Buy and Sell Stock with Cooldown](https://github.com/hitzzc/go-leetcode/tree/master/best_time_to_buy_and_sell_stock_with_cooldown)
 #### [312. Burst Balloons](https://github.com/hitzzc/go-leetcode/tree/master/burst_balloons)
 #### [313. Super Ugly Number](https://github.com/hitzzc/go-leetcode/tree/master/super_ugly_number)
+#### [315. Count of Smaller Numbers After Self](https://github.com/hitzzc/go-leetcode/tree/master/count_of_smaller_numbers_after_self)
 
 
 

count_of_smaller_numbers_after_self/count_of_smaller_numbers_after_self.cpp

@@ -0,0 +1,41 @@
+class binary_index_tree {
+	vector<int> array;
+public:
+	binary_index_tree(int n): array(n+1, 0) {}
+	int lowbit(int x) {
+		return x & (-x);
+	}
+	int sum(int idx) {
+		int ret = 0;
+		while (idx > 0) {
+			ret += array[idx];
+			idx -= lowbit(idx);
+		}
+		return ret;
+	}
+	void add(int idx, int val) {
+		while (idx < array.size()) {
+			array[idx] += val;
+			idx += lowbit(idx);
+		}
+	}
+}; 
+
+class Solution {
+public:
+	vector<int> countSmaller(vector<int>& nums) {
+		vector<int> sorted = nums;
+		sort(sorted.begin(), sorted.end());
+		unordered_map<int, int> mapping;
+		for (int i = 0; i < sorted.size(); ++i) {
+			mapping[sorted[i]] = i+1;
+		}
+		binary_index_tree my_bit(nums.size());
+		vector<int> ret(nums.size());
+		for (int i = nums.size()-1; i >=0; --i) {
+			ret[i] = my_bit.sum(mapping[nums[i]]-1);
+			my_bit.add(mapping[nums[i]], 1);
+		}
+		return ret;
+	}
+};