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BuildTree105.java
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BuildTree105.java
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package Leetcode;
import java.util.Deque;
import java.util.HashMap;
import java.util.LinkedList;
import java.util.Map;
public class BuildTree105 {
/**
Method: recursion
TIME: O(N)
SPACE:O(N)
*/
public TreeNode buildTree(int[] preorder, int[] inorder) {
if(preorder == null || preorder.length == 0) {
return null;
}
TreeNode root = new TreeNode(preorder[0]);
Deque<TreeNode> stack = new LinkedList<>();
stack.push(root);
int inorderIndex = 0;
for(int i=1; i<preorder.length; i++) {
int preVal = preorder[i];
TreeNode node = stack.peek();
if(node.val != inorder[inorderIndex]) {
node.left = new TreeNode(preVal);
stack.push(node.left);
} else{
while(!stack.isEmpty() && stack.peek().val == inorder[inorderIndex]) {
node = stack.pop();
inorderIndex ++;
}
node.right = new TreeNode(preVal);
stack.push(node.right);
}
}
return root;
}
/**
Method: iteration
TIME: O(N)
SPACE:O(N)
*/
Map<Integer, Integer> indexMap;
public TreeNode buildMyTree(int[] preorder, int[] inorder, int leftPre, int rightPre, int leftIn, int rightIn) {
if(leftPre>rightPre) {
return null;
}
int rootPre = leftPre;
int rootIn = indexMap.get(preorder[rootPre]);
TreeNode root = new TreeNode(preorder[rootPre]);
int size_left = rootIn - leftIn;
root.left = buildMyTree(preorder, inorder, leftPre+1, leftPre + size_left, leftIn, rootIn-1);
root.right = buildMyTree(preorder, inorder, leftPre + size_left+1, rightPre, rootIn+1, rightIn);
return root;
}
public TreeNode buildTree1(int[] preorder, int[] inorder) {
indexMap = new HashMap<>();
int n = preorder.length;
for(int i=0;i<n;i++) {
indexMap.put(inorder[i], i);
}
return buildMyTree(preorder, inorder, 0, n-1, 0, n-1);
}
//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;
}
}
}