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Hello everyone, the purpose of this blog is to record the summer vacation learning check-in, which will be organized into a column later. I mainly plan to finish learning data structure in the summer vacation, so I will post some blogs related to data structure implementation and some practice questions. I will use it for personal study and hope that everyone will support me. Please point out any shortcomings. Thank you.

1. Force buckle 115, minimum stack

. - LeetCode

Simply use an array to simulate a stack, but I am trying to save trouble here so the running speed is not very high, and there is a lot of room for optimization.

class MinStack {
 
    int[] el;
    int numsize;
 
    public MinStack() {
        el = new int[10000];
        int numsize = 0;
    }
 
    // private void grow(){
    // this.el=Arrays.copyof(el,2*el.lenth);
    // }
 
    public void push(int val) {
        // if (el.lenth == numsize) {
        // grow();
        // }
        el[numsize] = val;
        numsize++;
    }
 
    public int pop() {
        if (empty())
            return -1;
        return el[--numsize];
    }
 
    public int top() {
        return el[numsize - 1];
 
    }
 
    private boolean empty() {
        return numsize == 0;
    }
 
    public int getMin() {
        int num = el[0];
        for (int i = 0; i < numsize; i++) {
            if (el[i] < num)
                num = el[i];
        }
        return num;
    }
}
 
/**
 * Your MinStack object will be instantiated and called as such:
 * MinStack obj = new MinStack();
 * obj.push(val);
 * obj.pop();
 * int param_3 = obj.top();
 * int param_4 = obj.getMin();
 */

2. Pascal's Triangle

. - LeetCode

Analysis: In order to use the sequence table learned before, we use the sequence table to solve this problem. Use the sequence table to simulate a two-dimensional array. NoteThe two-dimensional array simulated by the sequence list cannot simply access elements by subscript

 
        class Solution {
    public List<List<Integer>> generate(int numRows) {
        List<List<Integer>> ret=new ArrayList<>();
        List<Integer> list=new ArrayList<>();
        list.add(1);
        ret.add(list);
        for(int i=1;i<numRows;i++){
            List<Integer> row=new ArrayList<>();
            row.add(1);
            List<Integer> a= ret.get(i-1);
            for (int j = 1; j < i; j++) {
                int val1=a.get(j);
                int val2=a.get(j-1);
                row.add(val1+val2);
            }
            row.add(1);
            ret.add(row);
        }
        return ret;
    }
}
 

3. 150-degree reverse Polish notation evaluation

Note: Polish notation is a formula that can be understood by computers.

. - LeetCode

Idea: Traverse the array and first determine whether the string is a number.If it is a number, convert it into a number and push it into the stack. Otherwise, take out two operands and take out the one after the "+", "-", "*" or "/" after the operator is used., the final answer is what is left in the stack.

class Solution {
    public int evalRPN(String[] tokens) {
        Stack<Integer> st = new Stack();
        for (int i = 0; i < tokens.length; i++) {
            String str = tokens[i];
            if (o(str) == false) {
                int val = Integer.parseInt(str);
                st.push(val);
            } else {
                int val1 = st.pop();
                int val2 = st.pop();
                switch (str) {
                    case "+":
                        st.push(val2+val1);
                        break;
                    case "-":
                        st.push(val2-val1);
                        break;
                        case "*":
                        st.push(val2*val1);
                        break;
                        case "/":
                        st.push(val2/val1);
                        break;
                }
 
            }
        }
        return st.peek();
    }
 
 
 
        private boolean o (String s){
            if (s.equals("+") || s.equals("-") || s.equals("*") || s.equals("/")) {
 
                return true;
            }
            return false;
        }
    }
 

This is the end of this blog, thank you everyone