LeetCode -- Construct Binary Tree from Inorder and Postorder Traversal
题目描述:
Given inorder and postorder traversal of a tree, construct the binary tree.
Note:
You may assume that duplicates do not exist in the tree.
就是输入中序遍历(左右中)和后序遍历(左右中)序列,生成二叉树。
思路:
设iFrom, iTo 分别inorder的起始和终止索引; pFrom ,pTo为递归过程中postorder的起始和终止索引。
1. 每次取后序遍历的最后节点,作为当前根节点,即current = new TreeNode(postorder[len - 1])
2. 从inorder序列中找到postorder[len-1]的索引,记为index
3. 创建左子树: inorder的索引范围:[0,index) , postorder的索引范围:[pFrom, pFrom + 距离(index, iFrom) - 1)。
4. 创建右子树: inorder的索引范围: [index + 1, len), postorder 的索引范围: [pFrom + 距离(index, iFrom), pTo-1]。
终止条件:pFrom > pTo 或iFrom > iTo
实现代码:
Given inorder and postorder traversal of a tree, construct the binary tree.
Note:
You may assume that duplicates do not exist in the tree.
就是输入中序遍历(左右中)和后序遍历(左右中)序列,生成二叉树。
思路:
设iFrom, iTo 分别inorder的起始和终止索引; pFrom ,pTo为递归过程中postorder的起始和终止索引。
1. 每次取后序遍历的最后节点,作为当前根节点,即current = new TreeNode(postorder[len - 1])
2. 从inorder序列中找到postorder[len-1]的索引,记为index
3. 创建左子树: inorder的索引范围:[0,index) , postorder的索引范围:[pFrom, pFrom + 距离(index, iFrom) - 1)。
4. 创建右子树: inorder的索引范围: [index + 1, len), postorder 的索引范围: [pFrom + 距离(index, iFrom), pTo-1]。
终止条件:pFrom > pTo 或iFrom > iTo
实现代码:
/**
* 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[] inorder, int[] postorder)
{
TreeNode node = null;
var len = postorder.Length - 1;
Build(ref node, inorder, 0, len, postorder, 0, len);
return node;
}
private void Build(ref TreeNode current, int [] inorder, int iFrom, int iTo, int[] postorder, int pFrom, int pTo)
{
if(iFrom > iTo || pFrom > pTo){
return;
}
// set current
current = new TreeNode(postorder[pTo]);
// take the last one from post order , because it is the root
var pLast = postorder[pTo];
// find its index in inorder
var index = -1;
for(var i = iFrom;i <= iTo; i++){
if(inorder[i] == pLast){
index = i;
break;
}
}
// for left sub tree , inorder : [0, index) . postorder : [pFrom, pFrom + distance(index, iFrom) - 1)
Build(ref current.left, inorder, iFrom, index - 1, postorder, pFrom, pFrom + index - iFrom - 1);
// for right sub tree , inorder : [index + 1, len), postorder : [pFrom + distance(index, iFrom), pTo-1]
Build(ref current.right, inorder, index + 1, iTo ,postorder, pFrom + index - iFrom, pTo - 1);
}
}