构造二叉树

一、由中序和后序遍历序列构造二叉树

106. 从中序与后序遍历序列构造二叉树 - 力扣（LeetCode）

/*** 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> map;//方便后续查找node位置public TreeNode buildTree(int[] inorder, int[] postorder) {map=new HashMap<>();for (int i=0;i<inorder.length;i++) { map.put(inorder[i],i);//将中序遍历序列保存在map中}return findNode(inorder,0,inorder.length,postorder,0,postorder.length);}public TreeNode findNode(int[] inorder,int ibegin,int iend,int[] postorder,int pbegin,int pend) {if(ibegin>=iend||pbegin>=pend)return null;//区间不满足左闭右开int rootindex=map.get(postorder[pend-1]);//得到后序遍历序列最后一个元素在中序遍历序列的位置TreeNode node=new TreeNode(inorder[rootindex]);//构造节点int leftsize=rootindex-ibegin;//左中序长度node.left=findNode(inorder,ibegin,rootindex,postorder,pbegin,pbegin+leftsize);node.right=findNode(inorder,rootindex+1,iend,postorder,pbegin+leftsize,pend-1);return node;}
}

二、由前序和中序遍历序列构造二叉树

105. 从前序与中序遍历序列构造二叉树 - 力扣（LeetCode）

class Solution {Map<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 findNode(inorder,0,inorder.length,preorder,0,preorder.length);}public TreeNode findNode(int[] inorder,int ibegin,int iend,int[] preorder,int pbegin,int pend) {if(ibegin>=iend||pbegin>=pend)return null;int rootIndex=map.get(preorder[pbegin]);TreeNode node=new TreeNode(inorder[rootIndex]);int leftsize=rootIndex-ibegin;node.left=findNode(inorder,ibegin,rootIndex,preorder,pbegin+1,pbegin+leftsize+1);node.right=findNode(inorder,rootIndex+1,iend,preorder,pbegin+leftsize+1,pend);return node;}
}