889. Construct Binary Tree from Preorder and Postorder Traversal

Medium

Given two integer arrays, preorder and postorder where preorder is the preorder traversal of a binary tree of distinct values and postorder is the postorder traversal of the same tree, reconstruct and return the binary tree.

If there exist multiple answers, you can return any of them.

Example 1:

Input: preorder = [1,2,4,5,3,6,7], postorder = [4,5,2,6,7,3,1]
Output: [1,2,3,4,5,6,7]

Example 2:

Input: preorder = [1], postorder = [1]
Output: [1]

Constraints:

• 1 <= preorder.length <= 30

• 1 <= preorder[i] <= preorder.length

• All the values of preorder are unique.

• postorder.length == preorder.length

• 1 <= postorder[i] <= postorder.length

• All the values of postorder are unique.

• It is guaranteed that preorder and postorder are the preorder traversal and postorder traversal of the same binary tree.

解題

Runtime: 0 ms, faster than 100%

Memory Usage: 3.3 MB, less than 69.57%

/**
 * Definition for a binary tree node.
 * type TreeNode struct {
 *     Val int
 *     Left *TreeNode
 *     Right *TreeNode
 * }
 */
func constructFromPrePost(preorder []int, postorder []int) *TreeNode {
    // pre 中左右
    // post 左右中
    l := len(postorder)

    if l == 0 { return nil }
    if l == 1 { return &TreeNode{ Val: preorder[0] }}

    n := preorder[0]
    left := preorder[1]
    for i:=0; i<l; i++ {
        if postorder[i] == left {
            return &TreeNode{
                Val: n,
                Left: constructFromPrePost(preorder[1:i+2], postorder[:i+1]),
                Right: constructFromPrePost(preorder[i+2:], postorder[i+1: l-1]),
            }
        }
    }

    return nil
}