输入某二叉树的前序遍历和中序遍历的结果，请重建该二叉树。假设输入的前序遍历和中序遍历的结果中都不含重复的数字。
例如，给出
前序遍历 preorder = [3,9,20,15,7]
中序遍历 inorder = [9,3,15,20,7]
返回如下的二叉树：
3
/ \
9 20
/ \
15 7
class Solution {
HashMap<Integer,Integer> map;
public TreeNode buildTree(int[] preorder, int[] inorder) {
map = new HashMap<>();
for(int i=0;i<inorder.length;i++)
{
map.put(inorder[i], i);
}
return process(preorder, inorder, 0, preorder.length-1, 0, inorder.length-1);
}
public TreeNode process(int[] preorder,int[] inorder,int pre_left,int pre_right,int in_left,int in_right)
{
if(pre_left>pre_right)
return null;
int root_pre = pre_left;
int root_in = map.get(preorder[root_pre]);
TreeNode root = new TreeNode(preorder[root_pre]);
int size_left_tree = root_in - in_left;
root.left = process(preorder, inorder, pre_left+1, pre_left+size_left_tree, in_left, root_in-1);
root.right = process(preorder, inorder, pre_left+size_left_tree+1, pre_right,root_in+1, in_right);
return root;
}
}
原文：https://www.cnblogs.com/pionice/p/14674151.html