LeetCode-_Questions/105.java at main · rashyy17/LeetCode-_Questions · GitHub

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/**
 * 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;
 *     }
 * }
 */
class Solution {

    public TreeNode buildTree(int[] preorder, int[] inorder) {
        Map<Integer, Integer> inMap = new HashMap<>();

        for(int i = 0; i < inorder.length; i++) {
            inMap.put(inorder[i], i);
        }

        return build(preorder, 0, preorder.length - 1,
                     inorder, 0, inorder.length - 1,
                     inMap);
    }

    TreeNode build(int[] preorder, int preStart, int preEnd,
                   int[] inorder, int inStart, int inEnd,
                   Map<Integer, Integer> inMap) {

        if(preStart > preEnd || inStart > inEnd) return null;

        TreeNode root = new TreeNode(preorder[preStart]);

        int inRoot = inMap.get(root.val);
        int numsLeft = inRoot - inStart;

        root.left = build(preorder,
                          preStart + 1,
                          preStart + numsLeft,
                          inorder,
                          inStart,
                          inRoot - 1,
                          inMap);

        root.right = build(preorder,
                           preStart + numsLeft + 1,
                           preEnd,
                           inorder,
                           inRoot + 1,
                           inEnd,
                           inMap);

        return root;
    }
}