Leetcode讲解：剑指 Offer 07. 重建二叉树

原题：剑指 Offer 07. 重建二叉树

class Solution {
    private Map<Integer,Integer> indexMap;

    public TreeNode myBulidTree(int[] preorder , int[] inorder , int preorder_left , int preorder_right , int inorder_left , int inorder_right){//preorder_left表示当前树的前序遍历数组范围，inorde_left表示当前树的中序遍历数组范围
        if(preorder_left > preorder_right) return null;

        int preorder_root  = preorder_left;//preorder_root表示根节点在前序遍历数组中的位置
        int inorder_root = indexMap.get(preorder[preorder_root]);

        TreeNode root = new TreeNode(preorder[preorder_root]);
        int size_left_subtree = inorder_root - inorder_left;
        root.left = myBulidTree(preorder , inorder , preorder_left + 1 , preorder_left + size_left_subtree , inorder_left , inorder_root - 1);
        root.right = myBulidTree(preorder , inorder , preorder_left + size_left_subtree + 1 , preorder_right , inorder_root + 1 , inorder_right);
        return root;
    }

    public TreeNode buildTree(int[] preorder, int[] inorder) {
        int n = preorder.length;
        indexMap = new HashMap<>();
        for(int i = 0 ; i < n ; i++){
            indexMap.put(inorder[i] , i);
        }
        return myBulidTree(preorder , inorder , 0 , n - 1 , 0 , n -1 );
    }
}