lintcode-不同的二叉查詢樹-163
阿新 • • 發佈:2019-02-11
給出 n,問由 1...n 為節點組成的不同的二叉查詢樹有多少種?
您在真實的面試中是否遇到過這個題? 樣例給出n = 3,有5種不同形態的二叉查詢樹:
1 3 3 2 1
\ / / / \ \
3 2 1 1 3 2
/ / \ \
2 1 2 3
沒有什麼好辦法,就是找規律
The case for 3 elements example Count[3] = Count[0]*Count[2] (1 as root) + Count[1]*Count[1] (2 as root) + Count[2]*Count[0] (3 as root) Therefore, we can get the equation: Count[i] = ∑ Count[0...k] * [ k+1....i] 0<=k<i-1
class Solution { public: int numTrees(int n) { int dp[n+1]; fill(dp,dp+n+1,0); dp[0]=dp[1]=1; for(int i=2;i<=n;i++) for(int j=1;j<=i;j++) dp[i]+= dp[j-1]*dp[i-j]; return dp[n]; } };