空数组返回true？ 给定一个整数数组，如果每个元素都是 1 或 4，则返回 true

Question

Given an array of ints, return true if every element is a 1 or a 4. only14([1, 4, 1, 4]) → true only14([1, 4, 2, 4]) → false only14([1, 1]) → true

I came up with the following algorithm which returns all the expected answers, EXCEPT for an empty array which should return true :

public boolean only14(int[] nums) {

  boolean flag = false;

  for (int i = 0; i < nums.length; i++) {

    if (nums[i] == 1 || nums[i] == 4) {
      flag = true;
    } else {
      return false;
    }
  }
  return flag;
}

From the solutions, the following input should yield true , but I can't figure out why:

only14([]) → true

I understand the default value is 0, so what am I missing such that I should expect the return value of the boolean to be true ?

Answer 1

The following should work. It returns true unless there are any values that are not 1 or 4.

public boolean only14(int[] nums) {
  for (int i = 0; i < nums.length; i++) {
    int val = nums[i];
    if (val != 1 && val != 4) {
      return false;
    }
  }
  return true;
}

Answer 2

The empty set contains no values that aren't 1 or 4 , so it should be true . I would prefer a for-each loop like

public boolean only14(int[] nums) {
    for (int n : nums) {        // for each int n in nums
        if (n == 1 || n == 4) { // if it is 1 or 4 keep looping
            continue;
        }
        return false;           // it isn't 1 or 4, return false
    }
    return true;                // every value is 1 or 4.
}

In Java 8+, you could implement it with an IntStream like

public boolean only14(int[] nums) {
    return IntStream.of(nums).allMatch(x -> x == 1 || x == 4);
}

Answer 3

This code works fine:-

public boolean only14(int[] nums) {
  for(int i=0;i<=nums.length-1;i++){
     if (nums[i]!= 1 && nums[i]!= 4){
      return false;
    }
  }
  return true;
}