# Java recursive solution using map

• ``````public class Solution {
public TreeNode buildTree(int[] preorder, int[] inorder) {
int length = preorder.length;
if(length == 0) return null;
Map<Integer, Integer> map = new HashMap<>();
for(int i = 0; i < length; i++) {
map.put(inorder[i], i);
}
return build(preorder, inorder, 0, length -1, 0, length -1, map);

}

private TreeNode build(int[] preorder, int[] inorder, int prestart, int preend, int instart, int inend, Map<Integer, Integer> map) {
if(prestart > preend || instart > inend) return null;
if(prestart == preend) {
return new TreeNode(preorder[prestart]);
}
TreeNode node = new TreeNode(preorder[prestart]);
int mid = map.get(preorder[prestart]);
int prelength = mid - instart;
int postlength = inend - mid;
node.left = build(preorder, inorder, prestart + 1, prestart + prelength, instart, mid - 1, map);
node.right = build(preorder, inorder, prestart + prelength + 1, preend, mid + 1, inend, map);
return node;
}

}
``````

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