105. Construct Binary Tree from Preorder and Inorder Traversal

身体不好

 

 

 

class Solution {
    public TreeNode buildTree(int[] preorder, int[] inorder) {
        return helper(0, 0, inorder.length - 1, preorder, inorder);
        
    }
    
    public TreeNode helper(int preStart, int inStart, int inEnd, int[] preorder, int[] inorder) {
        if(preStart > preorder.length - 1 || inStart > inEnd) {
            return null;
        }
        TreeNode root = new TreeNode(preorder[preStart]);
        int rootIndex = 0;
        for(int i = inStart; i <= inEnd; i++) {  // i 不是 =0 是等于 = inStart
            if(root.val == inorder[i]) {
                rootIndex = i;
            }
        }
        root.left = helper(preStart + 1, inStart, rootIndex - 1, preorder, inorder);
        root.right = helper(preStart + rootIndex - inStart + 1, rootIndex + 1, inEnd, preorder, inorder);
        return root;
    }
}