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; } }