用递归比较方便。

public class Solution {    public TreeNode buildTree(int[] preorder, int[] inorder) {        if (preorder.length != inorder.length) return null;        return buildTree(preorder, 0, preorder.length,                inorder, 0, inorder.length);            }        private TreeNode buildTree(int[] preorder, int ps, int pe,        int[] inorder, int is, int ie)    {        if (pe-ps == 0) return null;        TreeNode node = new TreeNode(preorder[ps]);        int c = -1;        for (c = is; c < ie; c++)        {            if (inorder[c] == preorder[ps]) break;        }        // split into two parts        int l_ps = ps+1;        int l_pe = ps+1+c-is;        int l_is = is;        int l_ie = c;        node.left = buildTree(preorder, l_ps, l_pe, inorder, l_is, l_ie);        int r_ps = ps+1+c-is;        int r_pe = pe;        int r_is = c+1;        int r_ie = ie;        node.right = buildTree(preorder, r_ps, r_pe, inorder, r_is, r_ie);        return node;    }}