Leetcode 279. Perfect Squares

原题：

 

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.

 

解决方法：

 用DP来求解比较容易

- 首先需要i从1~sqrt(n)循环。

- 然后从i*i到n,我们可以更新dp：dp[j] = min(dp[j], dp[j-i2] +1)。

 

代码：

int numSquares(int n) {
        vector<int> dp(n+1, INT_MAX);
        dp[0] = 0;
        for(int i=1, i2;(i2=i*i)<= n; i++){
            for(int j= i2; j<=n; j++){
                dp[j] = min(dp[j], dp[j-i2] +1);
            }
        }
        return dp[n];
    }