105. 从前序与中序遍历序列构造二叉树

class Solution {
    HashMap inhm  = new HashMap();
    public TreeNode buildTree(int[] preorder, int[] inorder) {
        inhm.clear();
        for(int i = 0 ; i<preorder.length;i++) {
            inhm.put(inorder[i], i);
        }
        int n = preorder.length;
        return dfs(preorder, inorder , 0,n-1 , 0 , n-1);
    }
    public TreeNode dfs(int[]preorder,int[]inorder,int pl , int pr , int il ,int ir) {
        if(pl>pr)return null;
        TreeNode root = new TreeNode(preorder[pl]);
        int k = inhm.get(preorder[pl]);
        int len = k - il;
        root.left = dfs(preorder , inorder , pl+1 ,pl+len ,il ,k-1 );
        root.right = dfs(preorder , inorder , pl+len+1, pr, k+1, ir);
        return root;
    }
}

自己写的

class Solution {
    int index = 0;
    public TreeNode buildTree(int[] preorder, int[] inorder) {
        return createBuild(preorder,inorder,0,preorder.length-1);
    }
    public TreeNode createBuild(int []preorder,int[]inorder,int start ,int end){
        if(start>end)return null;
        int val = preorder[index++];
        TreeNode root = new TreeNode(val);
        for (int i = 0; i < inorder.length; i++) {
            if(inorder[i]==val){
                root.left = createBuild(preorder,inorder,start ,i-1);
                root.right = createBuild(preorder,inorder,i+1 , end);
                break;
            }
        }
        return root;
    }
}