the previous numbers

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本文介绍了一种算法，用于解决给定数组中每个元素找到其前面最近的小于它的元素的问题，并提供了一个C++实现示例。

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ExampleGiven arr = [2,3,6,1,5,5], return [2,2,3,1,1,1].
Explanation:
According to the meaning, find the first smaller element in front of each number.
Given arr = [6,3,1,2,5,10,9,15], return [6,3,1,1,2,5,5,9].
Explanation:
According to the meaning, find the first smaller element in front of each number.

class Solution {
public:
    /**
     * @param num: The arry you should handle
     * @return: Return the array
     */
    vector<int> getPreviousNumber(vector<int> &num) {
        // Write your code here
        stack<int> s;
        vector<int> res;
        for (int i = 0; i < num.size(); i++) {
            while (!s.empty() && s.top() >= num[i]) {
                s.pop();
            }
            if (s.empty()) {
                res.push_back(num[i]);
            } else {
                res.push_back(s.top());
            }
            s.push(num[i]);
        }
        return res;
    }
};