106. Construct Binary Tree from Inorder and Postorder Traversal

Given inorder and postorder traversal of a tree, construct the binary tree.

Note:
You may assume that duplicates do not exist in the tree.

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similar to 105

/**
* 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[] inorder, int[] postorder) {

if(inorder.length==0 || inorder.length!=postorder.length)    return null;
return helper(inorder, 0, inorder.length-1, postorder, 0, postorder.length-1);

}

private TreeNode helper(int[] in, int s1, int e1, int[] post, int s2, int e2){

if(s1>e1 || s2>e2)  return null;

int val = post[e2];
TreeNode cur = new TreeNode(val);

// find cur in inorder
int k = s1;
while(k<=e1 && in[k]!=val){
k++;
}

cur.left = helper(in, s1, k-1, post,s2, s2+ k-s1 -1);
cur.right = helper(in, k+1, e1, post, s2+k-s1, e2-1);

return cur;

}
}