Perfect Squares

• 实际上就是0-1背包问题的变种
• 给正数n, 找到最少的平方数是的他们的和等于n
• 12, 返回3，因为12 = 4 + 4 + 4;
• 13, 返回2，因为13 = 4 + 9.

class Solution {public:    int numSquares(int n) {        int dp[n+1];        for(int i = 0; i <= n; ++i){            dp[i] = i;        }        for(int i = 4; i <= n; ++i){            for(int j = 1; j <= sqrt(i); ++j){                if(j*j == i){                    dp[i] = 1;                    break;                }else                    dp[i] = min(dp[i], 1 + dp[i - j*j]);            }        }        return dp[n];    }};