leetcode 279: Perfect Squares

Given a positive integer n, find the least number of perfect square numbers (for example,

1, 4, 9, 16, ...

) which sum to n.

For example, given n =

12

, return

3

because

12 = 4 + 4 + 4

; given n =

13

, return

2

because

13 = 4 + 9

.

思路：

ans[12] = min( ans[12-9]+1, ans[12-4]+1, ans[12-1]+1 );

最初是用递归方法做，但超时了，因为涉及了许多重复计算；

后采用数组方式，记录中间计算结果，从1计算到n；

class Solution {
public:
int numSquares(int n) {
vector<int> ans(n+1,0);
ans[1] = 1;
for(int i=2;i<=n;i++)
{
int s = sqrt(i);
if(s*s == i)
ans[i] = 1;
else
{
int tmp = i;
for(int j=s;j>=1;j--)
{
int tmp2 = ans[i-j*j]+1;
if(tmp2<tmp)
tmp = tmp2;
}
ans[i] = tmp;
}
}
return ans
;
}
};

View Code