leet-code/unique-binary-search-trees/sol.go

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2021-11-08 12:56:51 +00:00
package main
// Time: O(n^2)
// Space: O(n)
func numTrees(n int) int {
if n < 3 {
return n
}
trees := map[int]int{
0: 1,
1: 1,
2: 2,
}
for c := 3; c <= n; c++ {
s := 0
for p := 0; p < c/2; p++ {
s += trees[p] * trees[c-p-1] * 2
}
if (c-1)%2 == 0 {
p := (c - 1) / 2
s += trees[p] * trees[p]
}
trees[c] = s
}
return trees[n]
}