求1+2+3+···+n

循环相加

public class Solution {
    public int Sum_Solution(int n) {
        int sum = n;
        for(int i = 1; i < n; i++){
            sum+=i;
        }
        return sum;
    }
}

递归

public class Solution {
    public int Sum_Solution(int n) {
        if(n == 1)
            return n;
        return n+Sum_Solution(n-1);
    }
}

变形递归

public int Sum_Solution(int n) {
        int sum = n; 
        boolean result = (n > 0) && ((sum += Sum_Solution(n-1)) > 0); 
        return sum;
    }