即使if语句为true，函数也会返回false

Question

Here is the problem: https://leetcode.com/problems/happy-number/

My solution:

static int count = 0; 
public static void Main(string[] args)
{
    Console.WriteLine(happyNumber(19));
    Console.ReadLine(); 
}

public static bool happyNumber(int  a)
{
    double result = 0;
    Stack<int> stapel = new Stack<int>(); 
    //Split the integer into single digits and save them in a stack
    while (a.ToString().Count() > 1)
    {
        stapel.Push(a % 10);
        a = a / 10;
    }
    if (a.ToString().Count() == 1)
    {
        stapel.Push(a);
    }
    // Add the square of the digits to get the result
    foreach (var item in stapel)
    {
        result += Math.Pow((double)item, 2);
    }
    // Check if it's a happy number
    if(result == 1.0)
    {
        return true;
    }
    // counter to stop if it is a endless loop
    else if(count < 100)
    {
        count++;
        happyNumber((int)result);
    }
    return false;
}

So the input 19 is a happy number and the if clauses is true in the 4th run. You can set a breakpoint at if(result == 1.0) to check it. So why my function returns false instead?

Answer 1

Your function is recursive, but you don't do anything with the result of the recursive call.

If you change:

happyNumber((int)result);

To:

return happyNumber((int)result);

Then your result for 19 is true . There are potentially other issues with the comparison of floating point numbers, but that's probably your main problem!

Answer 2

You're unnecessarily casting to a double. Make result an int rather than a double (or make it a long if you're concerned the result will be too large for an int ). Replace the call to Math.Pow with manually squaring item , like so:

result += item * item;

The reason that control flow does not enter the if(result == 1.0) block is due to the way floating-point values are internally represented. Testing for equality between double s is problematic, and so (in this scenario) you should probably just avoid using them entirely as they are unneeded.

You also have a recursive call here:

happyNumber((int)result);

However, that call does nothing, as you're not actually doing anything with the return value. Consider replacing that line with:

return happyNumber((int)result);

This will return the value of the recursive call, rather than just discarding it.

Answer 3

发生这种情况是因为您的happyNumber方法调用了自己（最后一行的第3行），然后从该调用中return true了return true行-但这仅将堆栈向上返回了happyNumber方法的堆栈....，然后单击了return false 。