剑指 Offer 07. 重建二叉树

剑指 Offer 07. 重建二叉树

class Solution {
    HashMap<Integer, Integer> map = new HashMap<>();

    public TreeNode buildTree(int[] preorder, int[] inorder) {
        for(int i = 0; i < inorder.length; i++) map.put(inorder[i], i);
        return buildTree(preorder, 0, preorder.length - 1, inorder, 0, inorder.length - 1);
    }

    TreeNode buildTree(int[] preorder, int pre_l, int pre_r, int[] inorder, int in_l, int in_r){
        if(pre_l > pre_r || in_l > in_r) return null;
        TreeNode root = new TreeNode(preorder[pre_l]);
        int root_ind = map.get(preorder[pre_l]), left_size = root_ind - in_l;

        root.left = buildTree(preorder, pre_l + 1, pre_l + left_size, inorder, in_l, root_ind - 1);
        root.right = buildTree(preorder, pre_l + left_size + 1, pre_r, inorder, root_ind + 1, in_r);

        return root;
    }
}

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