LeetCode [279] Perfect Squares

279. Perfect Squares

Medium

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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class Solution{ public: //bt: TLE int numSquares(int n){ int m = sqrt(n); vector<int> vec(m); for(int i=1; i<=m; ++i){ vec[i-1] = i*i; } int min_sz = INT_MAX; bt(vec, n, min_sz, m, 0, 0, 0); return min_sz; } void bt(vector<int> vec, int target, int &min_sz, int sz, int pos, int cur_sz, int sum){ if(sum==target){ min_sz = min(min_sz, cur_sz); }else if(pos<sz && sum<target && cur_sz<min_sz){ for(int i=pos; i<sz; ++i){ if(sum+vec[i]>target) return; bt(vec, target, min_sz, sz, i, cur_sz+1, sum+vec[i]); } } } //dp: 428 ms int numSquares(int n){ vector<int> dp(n+1); for(int i=0; i<=n; ++i){ dp[i] = i; if(i>3){ int r = sqrt(i); for(int j = r; j>=1; --j){ dp[i] = min(dp[i], 1+dp[i-j*j]); } } } return dp[n]; } };

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class Solution { public int numSquares(int n) { int[] dp = new int[n+1]; for(int i=1; i<=n; ++i){ dp[i] = Integer.MAX_VALUE; for(int j=1; j*j<=i; ++j){ dp[i] = Math.min(dp[i], 1+dp[i-j*j]); } } return dp[n]; } }