2025.6.18学习日记

class Solution {
    Map<Integer,Integer> in_idx = new HashMap<>();
    public TreeNode buildTree(int[] inorder, int[] postorder) {
        for(int i=0 ; i<inorder.length ; ++i){
            in_idx.put(inorder[i],i);
        }
        int n = inorder.length;
        return dfs(inorder,postorder,0,n-1,0,n-1);
    }
    private TreeNode dfs(int[] inorder,int[] postorder , int beg_in , int end_in,int beg_pos , int end_pos){
        //全部闭区间
        if(beg_in > end_in ||beg_pos > end_pos )return null;
        int rootIndex = in_idx.get(postorder[end_pos]);
        TreeNode root = new TreeNode(inorder[rootIndex]);
        int lenOfLeft = rootIndex - beg_in;//左子树个数
        int n=inorder.length;
        root.left = dfs(inorder,postorder,beg_in,beg_in+lenOfLeft-1,beg_pos,beg_pos+lenOfLeft-1);
        root.right = dfs(inorder,postorder,rootIndex+1,end_in,beg_pos+lenOfLeft,end_pos-1);
        return root;
    }
}