|
| 1 | +#!/usr/bin/python3 |
| 2 | +""" |
| 3 | +We run a preorder depth first search on the root of a binary tree. |
| 4 | +
|
| 5 | +At each node in this traversal, we output D dashes (where D is the depth of this |
| 6 | +node), then we output the value of this node. (If the depth of a node is D, the |
| 7 | +depth of its immediate child is D+1. The depth of the root node is 0.) |
| 8 | +
|
| 9 | +If a node has only one child, that child is guaranteed to be the left child. |
| 10 | +
|
| 11 | +Given the output S of this traversal, recover the tree and return its root. |
| 12 | +
|
| 13 | +
|
| 14 | +Example 1: |
| 15 | +Input: "1-2--3--4-5--6--7" |
| 16 | +Output: [1,2,5,3,4,6,7] |
| 17 | +
|
| 18 | +Example 2: |
| 19 | +Input: "1-2--3---4-5--6---7" |
| 20 | +Output: [1,2,5,3,null,6,null,4,null,7] |
| 21 | +
|
| 22 | +
|
| 23 | +Example 3: |
| 24 | +Input: "1-401--349---90--88" |
| 25 | +Output: [1,401,null,349,88,90] |
| 26 | +
|
| 27 | +
|
| 28 | +Note: |
| 29 | +The number of nodes in the original tree is between 1 and 1000. |
| 30 | +Each node will have a value between 1 and 10^9. |
| 31 | +""" |
| 32 | + |
| 33 | + |
| 34 | +# Definition for a binary tree node. |
| 35 | +classTreeNode: |
| 36 | +def__init__(self,x): |
| 37 | +self.val=x |
| 38 | +self.left=None |
| 39 | +self.right=None |
| 40 | + |
| 41 | +fromcollectionsimportOrderedDict |
| 42 | + |
| 43 | + |
| 44 | +classSolution: |
| 45 | +defrecoverFromPreorder(self,S:str)->TreeNode: |
| 46 | +""" |
| 47 | + map: node -> depth |
| 48 | + stack of pi (incompleted) |
| 49 | + """ |
| 50 | +depth=0 |
| 51 | +# parse |
| 52 | +n=len(S) |
| 53 | +i=0 |
| 54 | +root=None |
| 55 | +stk= [] |
| 56 | +whilei<n: |
| 57 | +ifS[i]=="-": |
| 58 | +depth+=1 |
| 59 | +i+=1 |
| 60 | +else: |
| 61 | +j=i |
| 62 | +whilej<nandS[j]!="-": |
| 63 | +j+=1 |
| 64 | + |
| 65 | +val=int(S[i:j]) |
| 66 | + |
| 67 | +# construct |
| 68 | +cur=TreeNode(val) |
| 69 | +ifdepth==0: |
| 70 | +root=cur |
| 71 | +stk= [(depth,root)] |
| 72 | +else: |
| 73 | +assertstk |
| 74 | +whilestk[-1][0]!=depth-1: |
| 75 | +stk.pop() |
| 76 | + |
| 77 | +_,pi=stk[-1] |
| 78 | +ifnotpi.left: |
| 79 | +pi.left=cur |
| 80 | +elifnotpi.right: |
| 81 | +pi.right=cur |
| 82 | +stk.pop() |
| 83 | +else: |
| 84 | +raise |
| 85 | +stk.append((depth,cur)) |
| 86 | + |
| 87 | +depth=0 |
| 88 | +i=j |
| 89 | + |
| 90 | +returnroot |
| 91 | + |
| 92 | +defrecoverFromPreorder_error(self,S:str)->TreeNode: |
| 93 | +""" |
| 94 | + map: node -> depth |
| 95 | + stack of pi (incompleted) |
| 96 | + """ |
| 97 | +depth=0 |
| 98 | +depths=OrderedDict() |
| 99 | +# parse |
| 100 | +n=len(S) |
| 101 | +i=0 |
| 102 | +whilei<n: |
| 103 | +ifS[i]=="-": |
| 104 | +depth+=1 |
| 105 | +i+=1 |
| 106 | +else: |
| 107 | +j=i |
| 108 | +whilej<nandS[j]!="-": |
| 109 | +j+=1 |
| 110 | + |
| 111 | +val=int(S[i:j]) |
| 112 | +depths[val]=depth |
| 113 | +depth=0 |
| 114 | +i=j |
| 115 | + |
| 116 | +# construct |
| 117 | +stk= [] |
| 118 | +root=None |
| 119 | +fork,vindepths.items(): |
| 120 | +cur=TreeNode(k) |
| 121 | +ifv==0: |
| 122 | +root=cur |
| 123 | +stk= [root] |
| 124 | +else: |
| 125 | +assertstk |
| 126 | +whiledepths[stk[-1].val]!=v-1: |
| 127 | +stk.pop() |
| 128 | + |
| 129 | +ifnotstk[-1].left: |
| 130 | +stk[-1].left=cur |
| 131 | +elifnotstk[-1].right: |
| 132 | +stk[-1].right=cur |
| 133 | +stk.pop() |
| 134 | +else: |
| 135 | +raise |
| 136 | +stk.append(cur) |
| 137 | + |
| 138 | +returnroot |
| 139 | + |
| 140 | + |
| 141 | +if__name__=="__main__": |
| 142 | +assertSolution().recoverFromPreorder("5-4--4") |
| 143 | +assertSolution().recoverFromPreorder("1-2--3--4-5--6--7") |