construct-binary-tree-from-preorder-and-inorder-traversal

/**
 * 给出一棵树的前序遍历和中序遍历，请构造这颗二叉树
 * 注意：
 * 可以假设树中不存在重复的节点
 */

/**
 * 给出一棵树的前序遍历和中序遍历，请构造这颗二叉树
 * 注意：
 * 可以假设树中不存在重复的节点
 */

public class Main55 {
    public static void main(String[] args) {
        int[] preorde = {1,2,4,5,3,6,7};
        int[] inorder = {4,2,5,1,6,3,7};
        System.out.println(Main55.buildTree(preorde, inorder));
    }

    public static class TreeNode {
        int val;
        TreeNode left;
        TreeNode right;
        TreeNode(int x) { val = x; }
    }

    public static TreeNode buildTree(int[] preorder, int[] inorder) {
        if(preorder==null||inorder==null||preorder.length!=inorder.length)
            return null;
        return build(preorder, inorder, 0, preorder.length - 1, 0, inorder.length - 1);
    }

    public static TreeNode build(int[] preorder, int[] inorder, int prestart, int preend, int instart, int inend){
        // System.out.println("prestart = " + prestart + " preend = " + preend + " instart = " + instart + " inend = " + inend);
        if(preend<prestart||inend<instart)
            return null;
        TreeNode root= new TreeNode(preorder[prestart]);
        //System.out.println("root.val = " + root.val);
        int size = 0;
        int i = instart;
        for(; i <= inend; i++){
            if(root.val == inorder[i]){
                size = i - instart;
                break;}
        }
        root.left = build(preorder, inorder, prestart + 1, prestart + size, instart, i - 1 );
        root.right = build(preorder, inorder, prestart + size + 1, preend , i + 1 ,inend );
        return root;
    }
}