BFS/DFS解決隱形圖/樹問題

例題1

https://leetcode-cn.com/problems/perfect-squares/

描述

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.

解析

class Solution {
public:
    unordered_set<int> sett;
    int numSquares(int n) {
        queue<int> q;
        q.push(0);
        int step=0;
        while(!q.empty()){
            step++;
            int cnt=q.size();
            while(cnt--){   //逐層入隊
                int num=q.front();
                q.pop();
                for(int i=1;i*i<=n-num;i++){
                    int a=num+i*i;
                    if(a>n) break;
                    if(a==n) return step;
                    if(sett.count(a)==0){  //剪枝
                        q.push(a);  //將平方數的和入隊以保存狀態
                        sett.insert(a);
                    }                    
                }
            }            
        }
        return 0;
    }
};

待更新~