Step num problem

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subject

Define what is step sum？
such as 680,680 + 68 + 6 = 754,680 Of step sum It's called 754
Give a positive number num, Judge whether it is a certain number step sum

Their thinking

a number X Its steps and are called a , a number Y Its steps and are called B .
First of all, I have a corollary , If Y>X, Its steps and B can only be greater than a .

Two points
If the sum of steps in the middle is less than the target , It's in 1-mid Two points in
If the sum of the two halves of the middle is greater than the target , It's in mid+1-target Do two points
If you lose two points , I didn't get it either 7268, Illustrate this 7268 Not any number of steps and
This ideological tradition comes from a noun called monotony , Just rely on monotony , It gives you a little bit of monotony sensitivity

Code

	public static boolean isStepSum(int stepSum) {
    
		int L = 0;
		int R = stepSum;
		int M = 0;
		int cur = 0;
		while (L <= R) {
    
			M = L + ((R - L) >> 1);
			cur = stepSum(M);
			if (cur == stepSum) {
    
				return true;
			} else if (cur < stepSum) {
    
				L = M + 1;
			} else {
    
				R = M - 1;
			}
		}
		return false;
	}

	public static int stepSum(int num) {
    
		int sum = 0;
		while (num != 0) {
    
			sum += num;
			num /= 10;
		}
		return sum;
	}

summary

We first use the tabulation method to find out if X Greater than Y, be X The steps and of are also greater than Y And this conclusion
With this conclusion , We found the monotonicity , We can use two points