二分查找及四种变体

二分查找

    public static int binarySearch(int[] arr, int val) {
      int left = 0;
      int right = arr.length - 1;
      while (left <= right) {
        int mid = left + (right - left) / 2;
        if (arr[mid] > val) {
          right = mid - 1;
        } else if (arr[mid] < val) {
          left = mid + 1;
        } else {
          return mid;
        }
      }
       return -1;
    }

下面都是由上面代码修改而来

查找第一个值等于 val 的元素

  public static int getFirst(int[] arr, int val) {
    int left = 0, right = arr.length - 1;
    while (left <= right) {
      int mid = left + (right - left) / 2;
      if (arr[mid] > val) {
        right = mid - 1;
      } else if (arr[mid] < val) {
        left = mid + 1;
      } else {
        if ((mid == 0) || (arr[mid - 1] < val)) return mid;
        else right = mid - 1;
      }
    }
    return -1;
  }

查找最后一个值等于 val 的元素

上面第三个分支改成

		if ((mid == arr.length - 1) || (arr[mid + 1] > val)) return mid;

查找第一个值大于等于 val 的元素

  public static int getFirstEqualGreaterThanVal(int[] arr, int val) {
    int left = 0, right = arr.length - 1;
    while (left <= right) {
      int mid = left + (right - left) / 2;
      if (arr[mid] >= val) {
        if ((mid == 0) || (arr[mid - 1] < val)) return mid;
        else right= mid - 1;
      } else {
        left = mid + 1;
      }
    }
    return -1;
  }

查找第一个小于等于val的元素

  public static int getFirstEqualSmallerThanVal(int[] arr, int val) {
    int left = 0, right = arr.length - 1;
    while (left <= right) {
      int mid = left + (right - left) / 2;
      if (arr[mid] > val) {
        right = mid - 1;
      } else {
        if ((mid == arr.length - 1) || arr[mid + 1] > val) return mid;
        else left = mid + 1;
      }
    }
    return -1;
  }