404. Sum of Left Leaves

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题目链接

Find the sum of all left leaves in a given binary tree.

Example:

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    3
   / \
  9  20
    /  \
   15   7

There are two left leaves in the binary tree, with values 9 and 15 respectively. Return 24.

如何确定左叶子节点，首先该节点必须是一个叶子节点，其次它必须位于其父节点的左子树上．当使用先序遍历时，该节点存在于左子树上且是一个叶子节点．

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class Solution {
public:
    int sumOfLeftLeaves(TreeNode* root) {
        return sumOfLeftLeavesCore(root);
    }
    int sumOfLeftLeavesCore(TreeNode* root){
        int sum = 0;
        stack<TreeNode*> st;
        TreeNode* cur = root;
        while(cur!=nullptr || !st.empty()){
            if(cur == nullptr){
                cur = st.top();
                st.pop();
                cur = cur->right;
            }else{
                st.push(cur);
                cur = cur->left;
                if(cur && cur->left == nullptr && cur->right == nullptr){
                    sum += cur->val;
                }
            }
        }
        return sum;
    }
};