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hashmap记录inorder值于index的对应然后dfs，preorder第一个为root然后通过map找到root的index，和inStart，inEnd比较的到左右的大小然后再在preorder里找到根据左右大小找到left的root和right的root，如此迭代 public TreeNode build(int pre, int inStart, int inEnd, int[] preorder, HashMap<Integer,Integer> map){ if(pre >= preorder.length || inStart > inEnd) { return null; } TreeNode root = new TreeNode(preorder[pre]); if(inStart == inEnd) { return root; } int index = map.get(preorder[pre]); int leftSize = index - inStart; int rightSize = inEnd - index; int leftRoot = pre + 1; int rightRoot = pre + leftSize + 1; root.left = build(leftRoot, inStart, index-1, preorder, map); root.right = build(rightRoot, index+1, inEnd, preorder, map); return root; }

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