带你读《图解算法小抄》十二、树（12）

import isPowerOfTwo from '../../../algorithms/math/is-power-of-two/isPowerOfTwo';
export default class SegmentTree {
  /**
   * @param {number[]} inputArray
   * @param {function} operation - binary function (i.e. sum, min)
* @param {number} operationFallback - operation fallback value (i.e. 0 for sum, Infinity for min)
   */
  constructor(inputArray, operation, operationFallback) {
    this.inputArray = inputArray;
    this.operation = operation;
    this.operationFallback = operationFallback;
    // Init array representation of segment tree.
    this.segmentTree = this.initSegmentTree(this.inputArray);
    this.buildSegmentTree();
  }
  /**
   * @param {number[]} inputArray
   * @return {number[]}
   */
  initSegmentTree(inputArray) {
    let segmentTreeArrayLength;
    const inputArrayLength = inputArray.length;
    if (isPowerOfTwo(inputArrayLength)) {
      // If original array length is a power of two.
      segmentTreeArrayLength = (2 * inputArrayLength) - 1;
    } else {
      // If original array length is not a power of two then we need to find
      // next number that is a power of two and use it to calculate
      // tree array size. This is happens because we need to fill empty children
      // in perfect binary tree with nulls.And those nulls need extra space.
      const currentPower = Math.floor(Math.log2(inputArrayLength));
      const nextPower = currentPower + 1;
      const nextPowerOfTwoNumber = 2 ** nextPower;
      segmentTreeArrayLength = (2 * nextPowerOfTwoNumber) - 1;
    }
    return new Array(segmentTreeArrayLength).fill(null);
  }
  /**
   * Build segment tree.
   */
  buildSegmentTree() {
    const leftIndex = 0;
    const rightIndex = this.inputArray.length - 1;
    const position = 0;
    this.buildTreeRecursively(leftIndex, rightIndex, position);
  }
  /**
   * Build segment tree recursively.
   *
   * @param {number} leftInputIndex
   * @param {number} rightInputIndex
   * @param {number} position
   */
  buildTreeRecursively(leftInputIndex, rightInputIndex, position) {
    // If low input index and high input index are equal that would mean
    // the we have finished splitting and we are already came to the leaf
    // of the segment tree. We need to copy this leaf value from input
    // array to segment tree.
    if (leftInputIndex === rightInputIndex) {
      this.segmentTree[position] = this.inputArray[leftInputIndex];
      return;
    }
    // Split input array on two halves and process them recursively.
    const middleIndex = Math.floor((leftInputIndex + rightInputIndex) / 2);
    // Process left half of the input array.
    this.buildTreeRecursively(leftInputIndex, middleIndex, this.getLeftChildIndex(position));
     // Process right half of the input array.
    this.buildTreeRecursively(middleIndex + 1, rightInputIndex, this.getRightChildIndex(position));
    // Once every tree leaf is not empty we're able to build tree bottom up using
    // provided operation function.
    this.segmentTree[position] = this.operation(
      this.segmentTree[this.getLeftChildIndex(position)],
      this.segmentTree[this.getRightChildIndex(position)],
    );
  }
  /**
   * Do range query on segment tree in context of this.operation function.
   *
   * @param {number} queryLeftIndex
   * @param {number} queryRightIndex
   * @return {number}
   */
  rangeQuery(queryLeftIndex, queryRightIndex) {
    const leftIndex = 0;
    const rightIndex = this.inputArray.length - 1;
    const position = 0;
    return this.rangeQueryRecursive(
      queryLeftIndex,
      queryRightIndex,
      leftIndex,
      rightIndex,
      position,
    );
  }
  /**
   * Do range query on segment tree recursively in context of this.operation function.
   *
   * @param {number} queryLeftIndex - left index of the query
   * @param {number} queryRightIndex - right index of the query
   * @param {number} leftIndex - left index of input array segment
   * @param {number} rightIndex - right index of input array segment
   * @param {number} position - root position in binary tree
   * @return {number}
   */
  rangeQueryRecursive(queryLeftIndex, queryRightIndex, leftIndex, rightIndex, position) {
    if (queryLeftIndex <= leftIndex && queryRightIndex >= rightIndex) {
      // Total overlap.
      return this.segmentTree[position];
    }
    if (queryLeftIndex > rightIndex || queryRightIndex < leftIndex) {
      // No overlap.
      return this.operationFallback;
    }
    // Partial overlap.
    const middleIndex = Math.floor((leftIndex + rightIndex) / 2);
    const leftOperationResult = this.rangeQueryRecursive(
      queryLeftIndex,
      queryRightIndex,
      leftIndex,
      middleIndex,
      this.getLeftChildIndex(position),
    );
    const rightOperationResult = this.rangeQueryRecursive(
      queryLeftIndex,
      queryRightIndex,
      middleIndex + 1,
      rightIndex,
      this.getRightChildIndex(position),
        );
    return this.operation(leftOperationResult, rightOperationResult);
  }
  /**
   * Left child index.
   * @param {number} parentIndex
   * @return {number}
   */
  getLeftChildIndex(parentIndex) {
    return (2 * parentIndex) + 1;
  }
  /**
   * Right child index.
   * @param {number} parentIndex
   * @return {number}
   */
  getRightChildIndex(parentIndex) {
    return (2 * parentIndex) + 2;
  }}