LeetCode_105.从前序与中序遍历序列构造二叉树
根据一棵树的前序遍历与中序遍历构造二叉树。
注意:
你可以假设树中没有重复的元素。
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
C#代码
/**
* Definition for a binary tree node.
* public class TreeNode {
* public int val;
* public TreeNode left;
* public TreeNode right;
* public TreeNode(int x) { val = x; }
* }
*/
public class Solution {
public TreeNode BuildTree(int[] preorder, int[] inorder) {
int index = 0;
return BuildTree(preorder, inorder, 0, preorder.Length - 1,ref index);
}
public TreeNode BuildTree(int[] preorder, int[] inorder, int left, int right, ref int index)
{
/*递归边界条件:1.两数组长度不一致;2.任意数组长度为0.*/
if (preorder.Length != inorder.Length || preorder.Length == 0) return null;
TreeNode node = new TreeNode(preorder[index]);//current node
/*找到当前节点值在inorder中索引*/
int l = left;
while (l >= left && l <= right)
{
if (inorder[l] == preorder[index]) break;
l++;
}
index += 1;
if (l - left > 0) node.left = BuildTree(preorder, inorder, left, l - 1, ref index);
if (right - l > 0) node.right = BuildTree(preorder, inorder, l + 1, right, ref index);
return node;
}
}