[leetcode]Construct Binary Tree from Preorder and Inorder Traversal - java

根据先序集合找到根，再根据根，在中序集合中找到索引号，采用递归的构建子树的方法

/**

 * Definition for binary tree

 * public class TreeNode {

 *     int val;

 *     TreeNode left;

 *     TreeNode right;

 *     TreeNode(int x) { val = x; }

 * }

 */

public class Solution {

     public TreeNode buildTree(int[] preorder, int[] inorder) {

        return gT(preorder, 0, preorder.length-1, inorder, 0, inorder.length-1);

    }



    private TreeNode gT(int[] preOrder, int pStart, int pEnd, int[] inOrder, int inStart, int inEnd){

        if(preOrder==null || inOrder==null || preOrder.length==0 || inOrder.length==0 || inStart>inEnd || pStart>pEnd){

            return null;

        }

        int childRootVal = preOrder[pStart];

        TreeNode childRoot = new TreeNode(childRootVal);

        int childRootInOrderIndex = -1;

        for(int i=inStart; i<=inEnd; i++){

            if(inOrder[i]==childRootVal){

                childRootInOrderIndex=i;

                break;

            }

        }

        if(childRootInOrderIndex==-1){

            return null;

        }

        int duration = childRootInOrderIndex-inStart;

        childRoot.left=gT(preOrder, pStart+1, pStart+duration, inOrder, inStart, childRootInOrderIndex-1);

        childRoot.right=gT(preOrder, pStart+duration+1,pEnd, inOrder, childRootInOrderIndex+1, inEnd);

        return childRoot;

    }

}