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

.

Credits:

Special thanks to @jianchao.li.fighter for adding this problem and creating all test cases.

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背包

public class Solution {
public int numSquares(int n) {
int [] a = new int [n+1];
int i = 1;
while(i*i<=n){
a[i*i]=1;
i++;
}
for(int b = 2;b<=n;b++){
i=1;
if(a[b]==0){a[b]=a[b-1]+1;}
while(i*i<b){
int x= 1+a[b-i*i];
if(x<a[b])a[b]=x;
i++;
}
}
return a
;
}
}