(BFS) 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.

Example 1:

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

Example 2:

Input: n = 13
Output: 2
Explanation: 13 = 4 + 9.
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这个题可以用BFS，不过要注意排除重复的。
C++代码：

class Solution {
public:
    int numSquares(int n) {
        if(n == 0)
            return 0;
        queue<pair<int,int> > q;
        q.push(make_pair(n,1));
        vector<bool> vis(n+1,false);  //用这个来避免把重复的数加进队列。
        vis[n] = true;
        while(!q.empty()){
            int num = q.front().first;
            int s = q.front().second;
            q.pop();
            if(num == 0)
                return 0;
            for(int i = 1; num - i*i >=0 ; i++){
                int ans = num - i*i;
                if(ans == 0)
                    return s;
                if(!vis[ans]){
                    q.push(make_pair(ans,s+1));
                    vis[ans] = true;
                }
            }
        }
        return 0;
    }
};