数据结构之搜索

1. 线性搜索

从头开始和要查找的元素作比较，知道找到目标元素位置。

2. 二分搜索

1. 将数组排序
2. 取数组中间值，用中间值和目标值作比较。
3. 如果选中值是待搜索值，那么算法执行完毕（值找到了）
4. 如果中间值大于目标值，中间值左侧的值，执行第2步和第3步
5. 如果中间值小于目标值，中间值右侧的值，执行第2步和第3步
   

2.1 迭代

function binarySearch(array, key, compareFn = defaultCompareFn) {
  array = quickSort(array, compareFn);
  let low = 0;
  let high = array.length - 1;
  while (lesserOrEquals(low, high)) {
    const mid = Math.floor((low + high) / 2);
    const compare = compareFn(array[mid], key);
    if (compare === COMPARE.LESS_THAN) {
      low = mid + 1;
    }
    else if (compare === COMPARE.BIGGER_THAN) {
      high = mid - 1;
    }
    else {
      return array[mid];
    }
  }
  return -1;
}

function lesserOrEquals(a, b, compareFn = defaultCompareFn) {
  const compare = compareFn(a, b);
  return compare === COMPARE.EQUALITY || compare === COMPARE.LESS_THAN;
}

2.2 递归

function binarySearch(array, key, compareFn = defaultCompareFn) {
  const low = 0;
  const high = array.length - 1;
  array = quick(array, key, compareFn);
  return array[binarySearchRecursive(array, key, low, high, compareFn)];
}

function binarySearchRecursive(array, key, low, high, compareFn) {
  if (low <= high) {
    const mid = Math.floor((low + high) / 2);
    const compare = compareFn(array[mid], key);
    if (compare === COMPARE.LESS_THAN) {
      return binarySearchRecursive(array, key, mid + 1, high, compareFn);
    }
    else if (compare === COMPARE.BIGGER_THAN) {
      return binarySearchRecursive(array, key, low, mid - 1, compareFn);
    }
    else {
      return mid;
    }
  }
  else {
    return -1;
  }
}