Leetcode 從前序與中序遍歷序列構造二叉樹
阿新 • • 發佈:2020-11-12
Leetcode 105
資料結構定義:
根據一棵樹的前序遍歷與中序遍歷構造二叉樹。 注意: 你可以假設樹中沒有重複的元素。 例如,給出 前序遍歷 preorder =[3,9,20,15,7] 中序遍歷 inorder = [9,3,15,20,7] 返回如下的二叉樹: 3 / \ 9 20 / \ 15 7 /** * Definition for a binary tree node. * public class TreeNode { * int val; * TreeNode left; * TreeNode right; * TreeNode(int x) { val = x; } * } */
遞迴寫法:
class Solution { public TreeNode buildTree(int[] preorder, int[] inorder) { if(Objects.isNull(preorder) || preorder.length == 0 || Objects.isNull(inorder) || inorder.length == 0){ return null; } TreeNode root = new TreeNode(preorder[0]); int index = returnIndex(inorder,preorder[0]); if(index > 0){ root.left = buildTree(Arrays.copyOfRange(preorder,1,index+1), Arrays.copyOfRange(inorder,0,index)); } if(index != -1 && index < inorder.length -1){ root.right = buildTree(Arrays.copyOfRange(preorder,index+1,preorder.length), Arrays.copyOfRange(inorder,index+1,inorder.length)); } return root; } private int returnIndex(int[] array,int num){ for(int i =0;i<array.length;i++){ if(array[i] == num){ return i; } } return -1; } }
加快取的遞迴寫法:
class Solution { /* * map 做一個快取 */ private Map<Integer,Integer> map =new HashMap<>(); private int[] preorder ; public TreeNode buildTree(int[] preorder, int[] inorder) { if(preorder == null || inorder == null || preorder.length <= 0 || inorder.length <=0){ return null; } this.preorder =preorder; for(int i= 0; i<inorder.length;i++){ map.put(inorder[i],i); } return helpRecursion(0,preorder.length-1,0,preorder.length-1); } private TreeNode helpRecursion(int preStart,int preEnd,int inStart,int inEnd){ if(preStart > preEnd || inStart > inEnd){ return null; } TreeNode root = new TreeNode(preorder[preStart]); int index = map.get(preorder[preStart]); int leftSum = index - inStart; root.left = helpRecursion(preStart+1,preStart+leftSum,inStart,inStart+leftSum); root.right = helpRecursion(preStart+leftSum+1,preEnd,inStart+leftSum+1,inEnd); return root; } }
迭代寫法:
class Solution {
/*
*思路參考:https://leetcode-cn.com/problems/construct-binary-tree-from-preorder-and-inorder-traversal/solution/cong-qian-xu-yu-zhong-xu-bian-li-xu-lie-gou-zao-9/
*
*/
public TreeNode buildTree(int[] preorder, int[] inorder) {
if(preorder == null || preorder.length <= 0
|| inorder == null || inorder.length <=0){
return null;
}
int inorderIndex = 0;
TreeNode root =new TreeNode(preorder[0]);
Stack<TreeNode> stack = new Stack<>();
stack.push(root);
for(int i = 1;i < preorder.length;i++){
TreeNode node = stack.peek();
int value =preorder[i];
if(node.val != inorder[inorderIndex]){
node.left = new TreeNode(value);
stack.push(node.left);
}else{
while(!stack.isEmpty() && stack.peek().val == inorder[inorderIndex]){
node = stack.pop();
inorderIndex ++;
}
node.right = new TreeNode(value);
stack.push(node.right);
}
}
return root;
}
}