LC 987. Vertical Order Traversal of a Binary Tree

Given a binary tree, return the vertical order traversal of its nodes values.

For each node at position (X, Y), its left and right children respectively will be at positions (X-1, Y-1) and (X+1, Y-1).

Running a vertical line from X = -infinity to X = +infinity, whenever the vertical line touches some nodes, we report the values of the nodes in order from top to bottom (decreasing Y coordinates).

If two nodes have the same position, then the value of the node that is reported first is the value that is smaller.

Return an list of non-empty reports in order of X coordinate.  Every report will have a list of values of nodes.

class Solution {
public:
  vector<vector<int>> verticalTraversal(TreeNode* root) {
    map<int,map<int,vector<int>>> mp;
    vector<vector<int>> ret;
    helper(root, 0, 0, mp);
    for(auto it=mp.begin(); it!=mp.end(); it++){
      vector<int> a;
      for(auto iit=it->second.rbegin(); iit!=it->second.rend(); iit++) {
        vector<int> tmp = iit->second;
        sort(tmp.begin(), tmp.end());
        for(auto x : tmp) a.push_back(x);
      }
      ret.push_back(a);
    }
    return ret;
  }
  void helper(TreeNode* root,int x,int y, map<int,map<int,vector<int>>>& mp){
    if(!root) return;
    mp[x][y].push_back(root->val);
    helper(root->left, x-1, y-1, mp);
    helper(root->right, x+1, y-1, mp);
  }
};