[leetCode刷题笔记]2017.04.13

357. Count Numbers with Unique Digits

public class Solution {
    public int countNumbersWithUniqueDigits(int n) {
        if (n < 1) {
            return 1;
        }
        int[] uni = new int[n];
        uni[0] = 10;
        int sum = 10;
        for (int i = 1; i < n; i++) {
            if (i == 1) {
                uni[i] = 9 * 9;
            }
            else if (9 - i + 1 > 0){
                uni[i] = uni[i - 1] * (9 - i + 1);
            }
            else {
                return sum;
            }
            sum += uni[i];
        }
        return sum;
        
    }
}

                                                                                        

367. Valid Perfect Square

被超出int范围的数坑了，所以用long。

public class Solution {
    public boolean isPerfectSquare(int num) {
        long lnum = num;
        if (num < 0) return false;
        long i = 0;
        while (true) {
            if (i * i == lnum) return true;
            if (i * i > lnum) return false;
            i++;
        } 
        
    }
}