Construct Binary Tree from Preorder and Inorder Traversal.java

class Solution {
    Map<Integer, Integer> inMap;
    int curIndex = 0;
    int[] preOrder;
    public TreeNode buildTree(int[] preorder, int[] inorder) {
        preOrder = preorder;
        inMap = new HashMap<>();
        for(int i=0; i<inorder.length; i++) {
            inMap.put(inorder[i], i);
        }
        return dfs(0, preorder.length-1);
    }
    
    public TreeNode dfs(int s, int e){
        if(s > e) return null;
        int curNode = preOrder[curIndex++];
        TreeNode root  = new TreeNode(curNode);
        int inRoot = inMap.get(curNode);
        root.left = dfs(s, inRoot-1);
        root.right = dfs(inRoot+1, e);
        return root;
    }
}