Expression Tree Build

Description:

[ - ]
/          \
[ * ]              [ / ]
/     \           /         \
[ 2 ]  [ 6 ]      [ + ]        [ + ]
/    \       /      \
[ 23 ][ 7 ] [ 1 ]   [ 2 ] .

Solution:

/**
* 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) {
auto sz = (int)expression.size();
if (sz == 0) return nullptr;
stack<ExpressionTreeNode*> ops;
stack<ExpressionTreeNode*> nums;
unordered_map<string, int> cache;
cache["+"] = cache["-"] = 1;
cache["*"] = cache["/"] = 2;
for (int i = 0; i < sz; ++i) {
auto ch = expression[i];
if (ch == "+" || ch == "-" || ch == "*" || ch == "/") {
if (ops.empty()) {
ops.push(new ExpressionTreeNode(ch));
} else {
auto top = ops.top();
auto sym = top->symbol;
if (sym == "(" || cache[sym] < cache[ch]) {
ops.push(new ExpressionTreeNode(ch));
} else {
ops.pop();
auto num1 = nums.top();
nums.pop();
auto num2 = nums.top();
nums.pop();
top->left = num2;
top->right = num1;
nums.push(top);
--i;
}
}
} else if (ch == "(") {
ops.push(new ExpressionTreeNode(ch));
} else if (ch == ")") {
auto top = ops.top();
auto sym = top->symbol;
if (sym == "(") ops.pop();
else {
ops.pop();
auto num1 = nums.top();
nums.pop();
auto num2 = nums.top();
nums.pop();
top->left = num2;
top->right = num1;
nums.push(top);
--i;
}
} else {
nums.push(new ExpressionTreeNode(ch));
}
}
while (!ops.empty()) {
auto op = ops.top();
ops.pop();
auto num1 = nums.top();
nums.pop();
auto num2 = nums.top();
nums.pop();
op->left = num2;
op->right = num1;
nums.push(op);
}
return nums.empty() ? nullptr : nums.top();
}
};