LeetCode - 357 - Count Numbers with Unique Digits
2017-07-17 10:35
357 查看
Given a non-negative integer n, count all numbers with unique digits, x, where 0 ≤ x < 10n.
Example:
Given n = 2, return 91. (The answer should be the total numbers in the range of 0 ≤ x < 100, excluding
最开始没看懂题(最近总是被题意杀.......QAQ)独特的数的意思是,x各个位上的数字不相同。
n = 0, ans = 1; (这个数是0)
n = 1, ans = 0 ~ 9 = 10;
n = 2, ans = 9 * 9 + 10 = 81; (十位选取1~9,那么个位与十位不相同的数还有9个,则为9 * 9)
n = 3, ans = 9 * 9 * 8 + f(n=2) + f(n=1);
n = 4, ans = 9 * 9 * 8 * 7 + f(3) + f(2) + f(1);
……
n = 10, ans = 9 * 9 * 8 * 7 * …… * 1 + f(9) + f(8) + …… + f(1);
n > 10, ans = 0; (n大于10后无法保证各个位上数字不等)
时间复杂度O(n),空间复杂度O(n),也可以将空间复杂度降到O(1),只存两个数。
class Solution {
public:
int countNumbersWithUniqueDigits(int n) {
vector<int> ans(13, 0);
ans[0] = 1;
ans[1] = 10;
ans[2] = 9 * 9;
int cnt = 8;
for (int i = 3; i <= 10; ++i) {
ans[i] = ans[i-1] * cnt;
cnt--;
}
for (int i = 2; i <= 10; ++i) {
ans[i] += ans[i-1];
}
if (n > 10) return ans[10];
return ans
;
}
};
Example:
Given n = 2, return 91. (The answer should be the total numbers in the range of 0 ≤ x < 100, excluding
[11,22,33,44,55,66,77,88,99])
最开始没看懂题(最近总是被题意杀.......QAQ)独特的数的意思是,x各个位上的数字不相同。
n = 0, ans = 1; (这个数是0)
n = 1, ans = 0 ~ 9 = 10;
n = 2, ans = 9 * 9 + 10 = 81; (十位选取1~9,那么个位与十位不相同的数还有9个,则为9 * 9)
n = 3, ans = 9 * 9 * 8 + f(n=2) + f(n=1);
n = 4, ans = 9 * 9 * 8 * 7 + f(3) + f(2) + f(1);
……
n = 10, ans = 9 * 9 * 8 * 7 * …… * 1 + f(9) + f(8) + …… + f(1);
n > 10, ans = 0; (n大于10后无法保证各个位上数字不等)
时间复杂度O(n),空间复杂度O(n),也可以将空间复杂度降到O(1),只存两个数。
class Solution {
public:
int countNumbersWithUniqueDigits(int n) {
vector<int> ans(13, 0);
ans[0] = 1;
ans[1] = 10;
ans[2] = 9 * 9;
int cnt = 8;
for (int i = 3; i <= 10; ++i) {
ans[i] = ans[i-1] * cnt;
cnt--;
}
for (int i = 2; i <= 10; ++i) {
ans[i] += ans[i-1];
}
if (n > 10) return ans[10];
return ans
;
}
};
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