[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++; } } }