279 - Perfect Squares

#medium

Given an integer n, return the least number of perfect square numbers that sum to n.

A perfect square is an integer that is the square of an integer; in other words, it is the product of some integer with itself. For example, 1, 4, 9, and 16 are perfect squares while 3 and 11 are not.

Example 1:
Input: n = 12
Output: 3
Explanation: 12 = 4 + 4 + 4.

Example 2:
Input: n = 13
Output: 2
Explanation: 13 = 4 + 9.

class Solution {
public:
    int numSquares(int n) {
        // 使用bfs
        queue<int> que;
        unordered_set<int> visited;
        que.push( 0 );
        int result = 0;
        while ( !que.empty() ) {
            int que_size = que.size();
            result++;
            for ( int i = 0; i < que_size; i++ ) {
                int cur = que.front();
                que.pop();
                for ( int j = 1; cur < n; j++ ) {
                    int sum = cur + j*j;
                    if ( sum == n ) return result;
                    else if ( sum > n ) break;
                    else {
                        if ( !visited.count(sum) ) {
                            que.push( sum );
                            visited.insert( sum );
                        }
                    }
                }

            }

        }

        return result;
    }
};