题目链接:https://leetcode.cn/problems/construct-binary-tree-from-preorder-and-inorder-traversal/
/**
* Definition for a binary tree node.
* public class TreeNode {
* int val;
* TreeNode left;
* TreeNode right;
* TreeNode() {}
* TreeNode(int val) { this.val = val; }
* TreeNode(int val, TreeNode left, TreeNode right) {
* this.val = val;
* this.left = left;
* this.right = right;
* }
* }
*/
class Solution {
Map<Integer, Integer> inorderMap = new HashMap<>();
public TreeNode buildTree(int[] preorder, int[] inorder) {
if (preorder == null || inorder == null) return null;
for (int i = 0; i < inorder.length; i++) {
inorderMap.put(inorder[i], i);
}
TreeNode root = nodeBuild(preorder, 0, preorder.length, inorder, 0, inorder.length);
return root;
}
public TreeNode nodeBuild(int[] preorder, int preStart, int lenPreorder, int[] inorder, int inStart, int lenInorder) {
if (preStart >= lenPreorder || inStart >= lenInorder) {
return null;
}
int rootIndex = inorderMap.get(preorder[preStart]);
TreeNode root = new TreeNode(preorder[preStart]);
root.left = nodeBuild(preorder, preStart + 1, preStart +1 + rootIndex - inStart, inorder, inStart, rootIndex);
root.right = nodeBuild(preorder, preStart + rootIndex - inStart + 1, lenPreorder, inorder, rootIndex + 1, lenInorder);
return root;
}
}