/*** Definition for a binary tree node.* public class TreeNode {*     int val;*     TreeNode left;*     TreeNode right;*     TreeNode() {}*     TreeNode(int val) { this.val = val; }*     TreeNode(int val, TreeNode left, TreeNode right) {*         this.val = val;*         this.left = left;*         this.right = right;*     }* }*/// 解法一： 很好理解但太慢了
// class Solution {
//     public TreeNode buildTree(int[] inorder, int[] postorder) {
//         if(postorder.length==0){
//             return null;
//         }
//         TreeNode root = new TreeNode(postorder[postorder.length-1]);
//         if(postorder.length==1){
//             return root;
//         }
//         // 寻找切割点
//         int idx;
//         for(idx = 0; idx<inorder.length; idx++){
//             if(inorder[idx] == root.val){
//                 break;
//             }
//         }
//         // 递归子数组
//         root.left = buildTree(Arrays.copyOfRange(inorder,0,idx), Arrays.copyOfRange(postorder,0,idx));
//         root.right = buildTree(Arrays.copyOfRange(inorder,idx+1,inorder.length), Arrays.copyOfRange(postorder,idx,postorder.length-1));//         return root;
//     }
// } // 解法二
class Solution {Map<Integer,Integer> map; // 转换inorder，更快查找切割点索引public TreeNode buildTree(int[] inorder, int[] postorder) {map = new HashMap<>();for(int i = 0; i<inorder.length; i++){map.put(inorder[i],i);}return recur(inorder, 0, inorder.length, postorder, 0, postorder.length);}// 参数：中序数组-处理起点-处理终点，三者确定新的中序数组，，，后序同理private TreeNode recur(int[] inorder, int inStart, int inEnd, int[] postorder, int poStart, int poEnd){// 终止条件if (inStart == inEnd || poStart == poEnd) {// 相等了就到叶子了return null;}// 递归逻辑int rootVal = postorder[poEnd-1];TreeNode root = new TreeNode(rootVal);int rootIdx = map.get(rootVal);root.left = recur(inorder, inStart, rootIdx, postorder, poStart, poStart+rootIdx-inStart);root.right = recur(inorder, rootIdx+1, inEnd, postorder, poStart+rootIdx-inStart, poEnd-1);return root;}
}