Node.js) 백준 10090번: Counting Inversions

https://www.acmicpc.net/problem/10090

10090번: Counting Inversions

 

 

// https://www.acmicpc.net/problem/10090
// Counting Inversions
class Item {
	constructor(value = 0) {
		this.value = value;
	}
}

class SegmentTree {
	constructor(inputArray) {
		const inputArrayLength = inputArray.length;

		this.lg = Math.ceil(Math.log2(inputArrayLength));

		this.sz = 1 << this.lg;

		this.tree = Array.from(new Array(this.sz << 1), () => new Item());

		for (let i = 1; i <= inputArrayLength; i++) {
			this.tree[(i - 1) | this.sz] = new Item(inputArray[i - 1]);
		}

		for (let i = this.sz - 1; i > 0; i--) {
			this.tree[i] = this.merge(this.tree[i << 1], this.tree[(i << 1) | 1]);
			//  i << 1 은 왼쪽 자식 노드    i * 2
			// (i << 1) | 1 은 오른쪽 자식 노드 i * 2 + 1
		}
	}

	merge(A, B) {
		return new Item(A.value + B.value);
	}

	update(value, item) {
		return new Item(item.value + value);
	}

	apply(i, value) {
		this.tree[i] = this.update(value, this.tree[i]);
	}

	pull(i) {
		this.tree[i] = this.merge(this.tree[i << 1], this.tree[(i << 1) | 1]);
	}

	updatePoint(i, value) {
		i = (i - 1) | this.sz;
		this.apply(i, value);
		for (let j = 1; j <= this.lg; j++) this.pull(i >> j);
	}

	queryPoint(i) {
		i = (i - 1) | this.sz;
		return this.tree[i].value;
	}

	queryRange(l, r) {
		let L = new Item();
		let R = new Item();

		l = (l - 1) | this.sz;
		r = (r - 1) | this.sz;

		for (; l <= r; l >>= 1, r >>= 1) {
			if (l & 1) L = this.merge(L, this.tree[l++]);
			if (~r & 1) R = this.merge(this.tree[r--], R);
		}
		return this.merge(L, R).value;
	}
}
const input = require('fs').readFileSync('./dev/stdin').toString().trim().split('\n');
const N = +input[0];
const nums = input[1].split(' ');
const segmentTree = new SegmentTree(Array(N + 1).fill(0));

let answer = 0;
nums.forEach((num) => {
	segmentTree.updatePoint(+num, 1);
	answer += segmentTree.queryRange(+num + 1, N);
});
console.log(answer);