Given preorder and inorder traversal of a tree, construct the binary tree.
Note:
You may assume that duplicates do not exist in the tree.
思路分析:这题类似于LeetCode Construct Binary Tree from Inorder and Postorder Traversal,可以参考这篇题解,主要考察递归。
AC Code
/**
* Definition for binary tree
* public class TreeNode {
* int val;
* TreeNode left;
* TreeNode right;
* TreeNode(int x) { val = x; }
* }
*/
public class Solution {
public TreeNode buildTree(int[] preorder, int[] inorder) {
HashMap<Integer, Integer> inorderValueToIndexMap = new HashMap<Integer, Integer>();
for(int i = 0; i < inorder.length; i++){
inorderValueToIndexMap.put(inorder[i], i);
}
return buildTreeHelper(preorder, 0, preorder.length - 1, inorder, 0, inorder.length - 1, inorderValueToIndexMap);
}
public TreeNode buildTreeHelper(int [] preorder, int ps, int pe, int[] inorder, int is, int ie, HashMap<Integer, Integer> inorderValueToIndexMap){
if(ps > pe || is > ie) return null;
TreeNode root = new TreeNode(preorder[ps]);
int indexOfRootInorder = inorderValueToIndexMap.get(root.val);
int leftNum = indexOfRootInorder - is;
int rightNum = ie - indexOfRootInorder;
root.left = buildTreeHelper(preorder, ps + 1, ps + leftNum, inorder, indexOfRootInorder - leftNum, indexOfRootInorder -1, inorderValueToIndexMap);
root.right = buildTreeHelper(preorder, ps + leftNum + 1, ps + leftNum + rightNum, inorder, indexOfRootInorder + 1, indexOfRootInorder + rightNum, inorderValueToIndexMap);
return root;
}
}