java刷题--105从前序与中序遍历序列构造二叉树

Posted 2021-06-30 Anrys

java刷题--105从前序与中序遍历序列构造二叉树

• 题目
• 代码
• 结果

题目

代码

class Solution {
    public TreeNode buildTree(int[] preorder, int[] inorder) {
        return helper(0, 0, inorder.length - 1, preorder, inorder);
    }
    //前序pre 中序in
    public TreeNode helper(int preStart, int inStart, int inEnd, int[] preorder, int[] inorder) {
        if (preStart > preorder.length - 1 || inStart > inEnd) return null;
        TreeNode root = new TreeNode(preorder[preStart]);
        int inIndex = 0; // Index of current root in inorder
        for (int i = inStart; i <= inEnd; i++) {
            if (inorder[i] == root.val) {
                inIndex = i;
            }
        }
        root.left = helper(preStart + 1, inStart, inIndex - 1, preorder, inorder);
        root.right = helper(preStart + inIndex - inStart + 1, inIndex + 1, inEnd, preorder, inorder);
        return root;
    }
}

class Solution {
    public TreeNode buildTree(int[] preorder, int[] inorder) {
        if(preorder.length==0||inorder.length==0){
            return null; //返回必须是一个Treenode类型
        }
        TreeNode root=new TreeNode (preorder[0]);
        for(int i=0;i<preorder.length;i++){
            if(preorder[0]==inorder[i]){
                root.left=buildTree(Arrays.copyOfRange(preorder,1,i+1),
                                    Arrays.copyOfRange(inorder,0,i));
                root.right=buildTree(Arrays.copyOfRange(preorder,i+1,preorder.length),
                                    Arrays.copyOfRange(inorder,i+1,inorder.length));
                break;
            }
        }return root;
    }
}

结果