The structure of Expression Tree is a binary tree to evaluate certain expressions.
All leaves of the Expression Tree have an number string value. All non-leaves of the Expression Tree have an operator string value.
Now, given an expression array, build the expression tree of this expression, return the root of this expression tree.
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Clarification
See wiki:
Expression Tree
Example
For the expression(2*6-(23+7)/(1+2))(which can be represented by ["2" "*" "6" "-" "(" "23" "+" "7" ")" "/" "(" "1" "+" "2" ")"]).
The expression tree will be like
[ - ]
/ \
[ * ] [ / ]
/ \ / \
[ 2 ] [ 6 ] [ + ] [ + ]
/ \ / \
[ 23 ][ 7 ] [ 1 ] [ 2 ] .
After building the tree, you just need to return root node[-].
/**
* Definition of ExpressionTreeNode:
* class ExpressionTreeNode {
* public:
* string symbol;
* ExpressionTreeNode *left, *right;
* ExpressionTreeNode(string symbol) {
* this->symbol = symbol;
* this->left = this->right = NULL;
* }
* }
*/
class Solution {
public:
/\*\*
\* @param expression: A string array
\* @return: The root of expression tree
\*/
ExpressionTreeNode\* build\(vector<string> &expression\) {
// write your code here
stack<ExpressionTreeNode\*> s1;
stack<string> s2;
for \(string& s : expression\) {
if \(s == "\("\) {
s2.push\(s\);
} else if \(s == "\)"\) {
while \(s2.top\(\) != "\("\) {
buildTree\(s1, s2\);
}
s2.pop\(\);
} else {
if \(!IsOperator\(s\)\) {
s1.push\(new ExpressionTreeNode\(s\)\);
} else {
while \(!s2.empty\(\) && Precedence\(s2.top\(\)\) >= Precedence\(s\)\) {
buildTree\(s1, s2\);
}
s2.push\(s\);
}
}
}
while \(!s2.empty\(\)\) {
buildTree\(s1, s2\);
}
if \(s1.empty\(\)\) {
return NULL;
}
return s1.top\(\);
}
void buildTree\(stack<ExpressionTreeNode\*>& s1, stack<string>& s2\) {
ExpressionTreeNode\* node = new ExpressionTreeNode\(s2.top\(\)\);
s2.pop\(\);
node->right = s1.top\(\); s1.pop\(\);
node->left = s1.top\(\); s1.pop\(\);
s1.push\(node\);
}
bool IsOperator\(string& s\) {
return s == "+" \|\| s == "-" \|\| s == "\*" \|\| s == "/";
}
int Precedence\(string& op\) {
if \(op == "+" \|\| op == "-"\) {
return 1;
}
if \(op == "\*" \|\| op == "/"\) {
return 2;
}
return 0;
}
};