根据先序和中序遍历构建二叉树

public class BuildBinaryTree {

    static class TreeNode {
        int val;
        TreeNode left = null;
        TreeNode right = null;

        TreeNode(int val) {
            this.val = val;
        }
    }

    // 根据先序和中序遍历构建二叉树
    // root用于preOrder[]，begin和end用于inOrder[]
    public static TreeNode buildTree(int[] preOrder, int root,
                                      int[] inOrder, int begin, int end) {
        if (begin > end) return null;
        TreeNode node = new TreeNode(preOrder[root]);
        int loc, cnt = 0;
        for (loc = begin; loc <= end; loc++) {
            cnt++;
            if (preOrder[root] == inOrder[loc])
                break;
        }
        node.left = buildTree(preOrder, root + 1, inOrder, begin, loc - 1);
        node.right = buildTree(preOrder, root + cnt, inOrder, loc + 1, end);
        return node;
    }
}